diff --git a/docs/metadata.rst b/docs/metadata.rst
--- a/docs/metadata.rst
+++ b/docs/metadata.rst
@@ -38,11 +38,6 @@
 
       <codemeta:url>www.url-example.com</codemeta:url>
 
-- **the external\_identifier** *MAY* be provided as an identifier
-
-- **the external\_identifier** *SHOULD* match the Slug external-identifier in
-  the header
-
 - **the description** of the software deposit *SHOULD* be provided
   [codemeta:description]: short or long description of the software
 
@@ -63,7 +58,6 @@
         <entry xmlns="http://www.w3.org/2005/Atom">
             <title>Awesome Compiler</title>
             <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
-            <external_identifier>1785io25c695</external_identifier>
             <updated>2017-10-07T15:17:08Z</updated>
             <author>some awesome author</author>
     </entry>
@@ -78,7 +72,6 @@
                  xmlns:codemeta="https://doi.org/10.5063/SCHEMA/CODEMETA-2.0">
             <title>Awesome Compiler</title>
             <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
-            <external_identifier>1785io25c695</external_identifier>
             <codemeta:id>1785io25c695</codemeta:id>
             <codemeta:url>origin url</codemeta:url>
             <codemeta:identifier>other identifier, DOI, ARK</codemeta:identifier>
@@ -140,7 +133,6 @@
         <title>Awesome Compiler</title>
         <client>hal</client>
         <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
-        <external_identifier>%s</external_identifier>
         <dcterms:identifier>hal-01587361</dcterms:identifier>
         <dcterms:identifier>doi:10.5281/zenodo.438684</dcterms:identifier>
         <dcterms:title xml:lang="en">The assignment problem</dcterms:title>
diff --git a/docs/specs/metadata_example.xml b/docs/specs/metadata_example.xml
--- a/docs/specs/metadata_example.xml
+++ b/docs/specs/metadata_example.xml
@@ -7,7 +7,6 @@
           <email>hal@ccsd.cnrs.fr</email>
         </author>
         <client>hal</client>
-        <external_identifier>hal-01243573</external_identifier>
         <codemeta:name>The assignment problem</codemeta:name>
         <codemeta:url>https://hal.archives-ouvertes.fr/hal-01243573</codemeta:url>
         <codemeta:identifier>other identifier, DOI, ARK</codemeta:identifier>
diff --git a/docs/specs/spec-meta-deposit.rst b/docs/specs/spec-meta-deposit.rst
--- a/docs/specs/spec-meta-deposit.rst
+++ b/docs/specs/spec-meta-deposit.rst
@@ -48,7 +48,6 @@
         <email>hal@ccsd.cnrs.fr</email>
       </author>
       <client>hal</client>
-      <external_identifier>hal-01243573</external_identifier>
       <codemeta:name>The assignment problem</codemeta:name>
       <codemeta:url>https://hal.archives-ouvertes.fr/hal-01243573</codemeta:url>
       <codemeta:identifier>other identifier, DOI, ARK</codemeta:identifier>
diff --git a/docs/specs/spec-sparse-deposit.rst b/docs/specs/spec-sparse-deposit.rst
--- a/docs/specs/spec-sparse-deposit.rst
+++ b/docs/specs/spec-sparse-deposit.rst
@@ -44,7 +44,6 @@
           <email>hal@ccsd.cnrs.fr</email>
         </author>
         <client>hal</client>
-        <external_identifier>hal-01243573</external_identifier>
         <codemeta:name>The assignment problem</codemeta:name>
         <codemeta:url>https://hal.archives-ouvertes.fr/hal-01243573</codemeta:url>
         <codemeta:identifier>other identifier, DOI, ARK</codemeta:identifier>
diff --git a/docs/user-manual.rst b/docs/user-manual.rst
--- a/docs/user-manual.rst
+++ b/docs/user-manual.rst
@@ -102,7 +102,6 @@
      <title>Verifiable online voting system</title>
      <client>belenios</client>
      <id>belenios-01243065</id>
-     <external_identifier>test-01243065</external_identifier>
      <codemeta:url>https://gitlab.inria.fr/belenios/belenios</codemeta:url>
      <codemeta:applicationCategory>test</codemeta:applicationCategory>
      <codemeta:keywords>Online voting</codemeta:keywords>
diff --git a/swh/deposit/api/common.py b/swh/deposit/api/common.py
--- a/swh/deposit/api/common.py
+++ b/swh/deposit/api/common.py
@@ -773,6 +773,18 @@
                 "If the body is empty, there is no metadata.",
             )
 
+        if (
+            "atom:external_identifier" in metadata
+            and metadata["atom:external_identifier"] != headers.slug
+        ):
+            # TODO: When clients stopped using it, raise this error
+            # even when they are equal.
+            raise DepositError(
+                BAD_REQUEST,
+                "The 'external_identifier' tag is deprecated, "
+                "the Slug header should be used instead.",
+            )
+
         # Determine if we are in the metadata-only deposit case
         try:
             swhid = parse_swh_reference(metadata)
@@ -781,20 +793,14 @@
                 PARSING_ERROR, "Invalid SWHID reference", str(e),
             )
 
-        if swhid is not None:
-            external_id = metadata.get("external_identifier", headers.slug)
-        else:
-            slug = headers.slug
-            if check_slug_is_present and not slug:
-                raise_missing_slug_error()
-
-            external_id = metadata.get("external_identifier", slug)
+        if swhid is None and check_slug_is_present and not headers.slug:
+            raise_missing_slug_error()
 
         deposit = self._deposit_put(
             request,
             deposit_id=deposit_id,
             in_progress=headers.in_progress,
-            external_id=external_id,
+            external_id=headers.slug,
         )
 
         if swhid is not None:
diff --git a/swh/deposit/tests/api/test_collection.py b/swh/deposit/tests/api/test_collection.py
--- a/swh/deposit/tests/api/test_collection.py
+++ b/swh/deposit/tests/api/test_collection.py
@@ -115,7 +115,7 @@
     response = authenticated_client.post(
         reverse(COL_IRI, args=[deposit_collection.name]),
         content_type="application/atom+xml;type=entry",
-        data=atom_dataset["entry-data0"] % deposit.external_id,
+        data=atom_dataset["entry-data0"],
         HTTP_SLUG=deposit.external_id,
     )
 
@@ -145,7 +145,7 @@
     response = authenticated_client.post(
         reverse(COL_IRI, args=[deposit_collection.name]),
         content_type="application/atom+xml;type=entry",
-        data=atom_dataset["entry-data0"] % deposit.external_id,
+        data=atom_dataset["entry-data0"],
         HTTP_SLUG=deposit.external_id,
     )
 
@@ -176,7 +176,7 @@
     response = authenticated_client.post(
         reverse(COL_IRI, args=[deposit_collection.name]),
         content_type="application/atom+xml;type=entry",
-        data=atom_dataset["entry-data0"] % deposit.external_id,
+        data=atom_dataset["entry-data0"],
         HTTP_SLUG=deposit.external_id,
     )
 
diff --git a/swh/deposit/tests/api/test_collection_post_atom.py b/swh/deposit/tests/api/test_collection_post_atom.py
--- a/swh/deposit/tests/api/test_collection_post_atom.py
+++ b/swh/deposit/tests/api/test_collection_post_atom.py
@@ -116,6 +116,29 @@
     assert response.status_code == status.HTTP_400_BAD_REQUEST
 
 
+def test_post_deposit_atom_with_external_identifier(
+    authenticated_client, deposit_collection, atom_dataset
+):
+    """Posting an atom entry without a slug header should return a 400
+
+    """
+    url = reverse(COL_IRI, args=[deposit_collection.name])
+
+    # when
+    response = authenticated_client.post(
+        url,
+        content_type="application/atom+xml;type=entry",
+        data=atom_dataset["error-with-external-identifier"],
+        # + headers
+        HTTP_IN_PROGRESS="false",
+        HTTP_SLUG="something",
+    )
+
+    print(response.content)
+    assert b"The &#39;external_identifier&#39; tag is deprecated" in response.content
+    assert response.status_code == status.HTTP_400_BAD_REQUEST
+
+
 def test_post_deposit_atom_unknown_collection(authenticated_client, atom_dataset):
     """Posting an atom entry to an unknown collection should return a 404
 
@@ -146,7 +169,7 @@
     with pytest.raises(Deposit.DoesNotExist):
         Deposit.objects.get(external_id=external_id)
 
-    atom_entry_data = atom_dataset["entry-data0"] % external_id
+    atom_entry_data = atom_dataset["entry-data0"]
 
     # when
     response = authenticated_client.post(
@@ -187,7 +210,7 @@
     with pytest.raises(Deposit.DoesNotExist):
         Deposit.objects.get(external_id=external_id)
 
-    atom_entry_data = atom_dataset["codemeta-sample"] % external_id
+    atom_entry_data = atom_dataset["codemeta-sample"]
     # when
     response = authenticated_client.post(
         reverse(COL_IRI, args=[deposit_collection.name]),
@@ -292,9 +315,7 @@
     deposit_requests = DepositRequest.objects.filter(deposit=deposit)
     assert len(deposit_requests) == 1
 
-    atom_entry_data = atom_dataset["entry-data-minimal"] % external_id.encode(
-        "utf-8"
-    )  # noqa
+    atom_entry_data = atom_dataset["entry-data-minimal"]
 
     for link in response_content["atom:link"]:
         if link["@rel"] == "http://purl.org/net/sword/terms/add":
@@ -312,7 +333,7 @@
     )
 
     # then
-    assert response.status_code == status.HTTP_201_CREATED
+    assert response.status_code == status.HTTP_201_CREATED, response.content
 
     response_content = parse_xml(BytesIO(response.content))
     deposit_id = int(response_content["swh:deposit_id"])
diff --git a/swh/deposit/tests/api/test_deposit_update.py b/swh/deposit/tests/api/test_deposit_update.py
--- a/swh/deposit/tests/api/test_deposit_update.py
+++ b/swh/deposit/tests/api/test_deposit_update.py
@@ -111,7 +111,7 @@
     """
     # given
     deposit = partial_deposit_with_metadata
-    raw_metadata0 = atom_dataset["entry-data0"] % deposit.external_id.encode("utf-8")
+    raw_metadata0 = atom_dataset["entry-data0"]
 
     requests_meta = DepositRequest.objects.filter(deposit=deposit, type="metadata")
     assert len(requests_meta) == 1
@@ -236,9 +236,7 @@
     )
 
     assert len(requests) == 2
-    expected_raw_meta0 = atom_dataset["entry-data0"] % (
-        deposit.external_id.encode("utf-8")
-    )
+    expected_raw_meta0 = atom_dataset["entry-data0"]
     # a new one was added
     assert requests[0].raw_metadata == expected_raw_meta0
     assert requests[1].raw_metadata == atom_entry
@@ -301,9 +299,7 @@
     )
 
     assert len(requests) == 1 + 1, "New deposit request archive got added"
-    expected_raw_meta0 = atom_dataset["entry-data0"] % (
-        deposit.external_id.encode("utf-8")
-    )
+    expected_raw_meta0 = atom_dataset["entry-data0"]
     # a new one was added
     assert requests[0].raw_metadata == expected_raw_meta0
     assert requests[1].raw_metadata == data_atom_entry
@@ -518,7 +514,7 @@
     """
     # given
     deposit = partial_deposit_with_metadata
-    raw_metadata0 = atom_dataset["entry-data0"] % deposit.external_id.encode("utf-8")
+    raw_metadata0 = atom_dataset["entry-data0"]
 
     requests_meta = DepositRequest.objects.filter(deposit=deposit, type="metadata")
     assert len(requests_meta) == 1
diff --git a/swh/deposit/tests/conftest.py b/swh/deposit/tests/conftest.py
--- a/swh/deposit/tests/conftest.py
+++ b/swh/deposit/tests/conftest.py
@@ -317,7 +317,7 @@
     response = authenticated_client.post(
         reverse(SE_IRI, args=[collection_name, deposit.id]),
         content_type="application/atom+xml;type=entry",
-        data=atom_dataset["entry-data0"] % deposit.external_id.encode("utf-8"),
+        data=atom_dataset["entry-data0"],
         HTTP_SLUG=deposit.external_id,
         HTTP_IN_PROGRESS="true",
     )
diff --git a/swh/deposit/tests/data/atom/codemeta-sample.xml b/swh/deposit/tests/data/atom/codemeta-sample.xml
--- a/swh/deposit/tests/data/atom/codemeta-sample.xml
+++ b/swh/deposit/tests/data/atom/codemeta-sample.xml
@@ -2,7 +2,6 @@
         <entry xmlns="http://www.w3.org/2005/Atom"
                xmlns:dcterms="http://purl.org/dc/terms/"
                xmlns:codemeta="https://doi.org/10.5063/SCHEMA/CODEMETA-2.0">
-            <external_identifier>%s</external_identifier>
             <dcterms:identifier>hal-01587361</dcterms:identifier>
             <dcterms:identifier>https://hal.inria.fr/hal-01587361</dcterms:identifier>
             <dcterms:identifier>https://hal.inria.fr/hal-01587361/document</dcterms:identifier>
diff --git a/swh/deposit/tests/data/atom/entry-data-minimal.xml b/swh/deposit/tests/data/atom/entry-data-minimal.xml
--- a/swh/deposit/tests/data/atom/entry-data-minimal.xml
+++ b/swh/deposit/tests/data/atom/entry-data-minimal.xml
@@ -1,4 +1,4 @@
 <?xml version="1.0"?>
 <entry xmlns="http://www.w3.org/2005/Atom">
-    <external_identifier>%s</external_identifier>
+    <title>Awesome Compiler</title>
 </entry>
diff --git a/swh/deposit/tests/data/atom/entry-data0.xml b/swh/deposit/tests/data/atom/entry-data0.xml
--- a/swh/deposit/tests/data/atom/entry-data0.xml
+++ b/swh/deposit/tests/data/atom/entry-data0.xml
@@ -3,7 +3,6 @@
     <title>Awesome Compiler</title>
     <client>hal</client>
     <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
-    <external_identifier>%s</external_identifier>
     <updated>2017-10-07T15:17:08Z</updated>
     <author>some awesome author</author>
     <applicationCategory>something</applicationCategory>
diff --git a/swh/deposit/tests/data/atom/entry-data2.xml b/swh/deposit/tests/data/atom/entry-data2.xml
--- a/swh/deposit/tests/data/atom/entry-data2.xml
+++ b/swh/deposit/tests/data/atom/entry-data2.xml
@@ -1,6 +1,5 @@
 <?xml version="1.0"?>
 <entry xmlns="http://www.w3.org/2005/Atom">
-  <external_identifier>some-external-id</external_identifier>
   <url>https://hal-test.archives-ouvertes.fr/some-external-id</url>
   <author>some awesome author</author>
 </entry>
diff --git a/swh/deposit/tests/data/atom/error-with-decimal.xml b/swh/deposit/tests/data/atom/error-with-decimal.xml
--- a/swh/deposit/tests/data/atom/error-with-decimal.xml
+++ b/swh/deposit/tests/data/atom/error-with-decimal.xml
@@ -3,7 +3,6 @@
   <title>Composing a Web of Audio Applications</title>
   <client>hal</client>
   <id>hal-01243065</id>
-  <external_identifier>hal-01243065</external_identifier>
   <codemeta:url>https://hal-test.archives-ouvertes.fr/hal-01243065</codemeta:url>
   <codemeta:applicationCategory>test</codemeta:applicationCategory>
   <codemeta:name/>
diff --git a/swh/deposit/tests/data/atom/error-with-external-identifier.xml b/swh/deposit/tests/data/atom/error-with-external-identifier.xml
new file mode 100644
--- /dev/null
+++ b/swh/deposit/tests/data/atom/error-with-external-identifier.xml
@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<entry xmlns="http://www.w3.org/2005/Atom" xmlns:codemeta="https://doi.org/10.5063/SCHEMA/CODEMETA-2.0">
+  <title>Composing a Web of Audio Applications</title>
+  <client>hal</client>
+  <id>hal-01243065</id>
+  <external_identifier>hal-01243065</external_identifier>
+</entry>
diff --git a/swh/deposit/tests/data/atom/metadata.xml b/swh/deposit/tests/data/atom/metadata.xml
--- a/swh/deposit/tests/data/atom/metadata.xml
+++ b/swh/deposit/tests/data/atom/metadata.xml
@@ -4,7 +4,6 @@
   <title>Composing a Web of Audio Applications</title>
   <client>hal</client>
   <id>hal-01243065</id>
-  <external_identifier>hal-01243065</external_identifier>
   <codemeta:url>https://hal-test.archives-ouvertes.fr/hal-01243065</codemeta:url>
   <codemeta:applicationCategory>test</codemeta:applicationCategory>
   <codemeta:keywords>DSP programming</codemeta:keywords>