diff --git a/internal/archive/archive.go b/internal/archive/archive.go
index 7c90a41d..6e7d054e 100644
--- a/internal/archive/archive.go
+++ b/internal/archive/archive.go
@@ -157,6 +157,47 @@ func (a *ubuntuArchive) Info(pkg string) (*PackageInfo, error) {
 	return info, nil
 }
 
+// setupIndexes fetches the release and package indexes of every suite and
+// component, registering the resulting indexes on the archive.
+func (a *ubuntuArchive) setupIndexes() error {
+	for _, suite := range a.options.Suites {
+		var release control.Section
+		for _, component := range a.options.Components {
+			index := &ubuntuIndex{
+				label:     a.options.Label,
+				version:   a.options.Version,
+				arch:      a.options.Arch,
+				suite:     suite,
+				component: component,
+				release:   release,
+				archive:   a,
+			}
+			if release == nil {
+				err := index.fetchRelease()
+				if err != nil {
+					return err
+				}
+				release = index.release
+				if !index.supportsArch(a.options.Arch) {
+					// Release does not support the specified architecture, do
+					// not add any of its indexes.
+					break
+				}
+				err = index.checkComponents(a.options.Components)
+				if err != nil {
+					return err
+				}
+			}
+			err := index.fetchIndex()
+			if err != nil {
+				return err
+			}
+			a.indexes = append(a.indexes, index)
+		}
+	}
+	return nil
+}
+
 const ubuntuURL = "http://archive.ubuntu.com/ubuntu/"
 const ubuntuOldReleasesURL = "http://old-releases.ubuntu.com/ubuntu/"
 const ubuntuPortsURL = "http://ports.ubuntu.com/ubuntu-ports/"
@@ -189,28 +230,37 @@ var proArchiveInfo = map[string]struct {
 	},
 }
 
-func archiveURL(pro, arch string, oldRelease bool) (string, *credentials, error) {
+// candidateArchiveURLs returns the candidate base URLs of the archive, in order of
+// preference, and the credentials used to access them, if any.
+//
+// Ubuntu releases are moved from the regular archive to
+// old-releases.ubuntu.com after their end of life, but not immediately:
+// until the move happens the release is only available from the regular
+// archive. For such releases both archives are returned, so that the
+// caller can fall back to the regular one when the release is not found
+// in the old-releases one.
+func candidateArchiveURLs(pro, arch string, oldRelease bool) ([]string, *credentials, error) {
 	if pro != "" {
 		archiveInfo, ok := proArchiveInfo[pro]
 		if !ok {
-			return "", nil, fmt.Errorf("invalid pro value: %q", pro)
+			return nil, nil, fmt.Errorf("invalid pro value: %q", pro)
 		}
 		url := archiveInfo.BaseURL
 		creds, err := findCredentials(url)
 		if err != nil {
-			return "", nil, err
+			return nil, nil, err
 		}
-		return url, creds, nil
+		return []string{url}, creds, nil
 	}
 
-	if oldRelease {
-		return ubuntuOldReleasesURL, nil, nil
+	current := ubuntuURL
+	if arch != "amd64" && arch != "i386" {
+		current = ubuntuPortsURL
 	}
-
-	if arch == "amd64" || arch == "i386" {
-		return ubuntuURL, nil, nil
+	if oldRelease {
+		return []string{ubuntuOldReleasesURL, current}, nil, nil
 	}
-	return ubuntuPortsURL, nil, nil
+	return []string{current}, nil, nil
 }
 
 func openUbuntu(options *Options) (Archive, error) {
@@ -224,58 +274,31 @@ func openUbuntu(options *Options) (Archive, error) {
 		return nil, fmt.Errorf("archive options missing version")
 	}
 
-	baseURL, creds, err := archiveURL(options.Pro, options.Arch, options.OldRelease)
+	candidates, creds, err := candidateArchiveURLs(options.Pro, options.Arch, options.OldRelease)
 	if err != nil {
 		return nil, err
 	}
 
-	archive := &ubuntuArchive{
-		options: *options,
-		cache: &cache.Cache{
-			Dir: options.CacheDir,
-		},
-		pubKeys: options.PubKeys,
-		baseURL: baseURL,
-		creds:   creds,
-	}
-
-	for _, suite := range options.Suites {
-		var release control.Section
-		for _, component := range options.Components {
-			index := &ubuntuIndex{
-				label:     options.Label,
-				version:   options.Version,
-				arch:      options.Arch,
-				suite:     suite,
-				component: component,
-				release:   release,
-				archive:   archive,
-			}
-			if release == nil {
-				err := index.fetchRelease()
-				if err != nil {
-					return nil, err
-				}
-				release = index.release
-				if !index.supportsArch(options.Arch) {
-					// Release does not support the specified architecture, do
-					// not add any of its indexes.
-					break
-				}
-				err = index.checkComponents(options.Components)
-				if err != nil {
-					return nil, err
-				}
-			}
-			err := index.fetchIndex()
-			if err != nil {
-				return nil, err
-			}
-			archive.indexes = append(archive.indexes, index)
+	// Try the candidate archives in order until the release is found.
+	for _, baseURL := range candidates {
+		archive := &ubuntuArchive{
+			options: *options,
+			cache:   &cache.Cache{Dir: options.CacheDir},
+			pubKeys: options.PubKeys,
+			baseURL: baseURL,
+			creds:   creds,
+		}
+		err := archive.setupIndexes()
+		if err == errNotFound {
+			// Release not in this archive, try the next candidate.
+			continue
 		}
+		if err != nil {
+			return nil, err
+		}
+		return archive, nil
 	}
-
-	return archive, nil
+	return nil, errNotFound
 }
 
 func (index *ubuntuIndex) fetchRelease() error {
diff --git a/internal/archive/archive_test.go b/internal/archive/archive_test.go
index 90e3f004..9045c24c 100644
--- a/internal/archive/archive_test.go
+++ b/internal/archive/archive_test.go
@@ -536,6 +536,82 @@ func (s *httpSuite) TestOpenUnmaintainedArchives(c *C) {
 	c.Assert(err, IsNil)
 }
 
+func (s *httpSuite) TestOpenOldReleaseFallback(c *C) {
+	s.prepareArchive("plucky", "25.04", "amd64", []string{"main"})
+
+	// The old-releases mirror 404s the release (it has not been physically
+	// moved yet); the current archive serves the prepared content.
+	do := func(req *http.Request) (*http.Response, error) {
+		if strings.HasPrefix(req.URL.String(), "http://old-releases.ubuntu.com/ubuntu/") {
+			s.requestResults = append(s.requestResults, requestResult{path: req.URL.Path, status: 404})
+			return &http.Response{
+				Body:       io.NopCloser(strings.NewReader("")),
+				StatusCode: 404,
+			}, nil
+		}
+		return s.Do(req)
+	}
+	restoreDo := archive.FakeDo(do)
+	defer restoreDo()
+
+	options := archive.Options{
+		Label:      "ubuntu",
+		Version:    "25.04",
+		Arch:       "amd64",
+		Suites:     []string{"plucky"},
+		Components: []string{"main"},
+		CacheDir:   c.MkDir(),
+		PubKeys:    []*packet.PublicKey{s.pubKey},
+		OldRelease: true,
+	}
+
+	testArchive, err := archive.Open(&options)
+	c.Assert(err, IsNil)
+
+	_, _, err = testArchive.Fetch("mypkg1")
+	c.Assert(err, IsNil)
+
+	// Exactly one 404 (the InRelease fetch from old-releases); all
+	// subsequent requests must be served by the current archive.
+	oldReleasesHits := 0
+	for _, r := range s.requestResults {
+		if r.status == 404 {
+			oldReleasesHits++
+		}
+	}
+	c.Assert(oldReleasesHits, Equals, 1)
+}
+
+func (s *httpSuite) TestOpenOldReleaseNotFound(c *C) {
+	// No candidate archive distributes the release: accept requests from
+	// any host and 404 them all.
+	s.base = ""
+	s.status = 404
+
+	options := archive.Options{
+		Label:      "ubuntu",
+		Version:    "25.04",
+		Arch:       "amd64",
+		Suites:     []string{"plucky"},
+		Components: []string{"main"},
+		CacheDir:   c.MkDir(),
+		PubKeys:    []*packet.PublicKey{s.pubKey},
+		OldRelease: true,
+	}
+
+	_, err := archive.Open(&options)
+	c.Assert(err, ErrorMatches, "cannot find archive data")
+
+	// Both candidates must have been tried, one InRelease fetch each.
+	suites := 0
+	for _, r := range s.requestResults {
+		if strings.HasSuffix(r.path, "/dists/plucky/InRelease") {
+			suites++
+		}
+	}
+	c.Assert(suites, Equals, 2)
+}
+
 type verifyArchiveReleaseTest struct {
 	summary string
 	pubKeys []*packet.PublicKey