diff --git a/mercurial/commit.py b/mercurial/commit.py
--- a/mercurial/commit.py
+++ b/mercurial/commit.py
@@ -390,7 +390,7 @@
                 meta[b"copy"] = cfname
                 meta[b"copyrev"] = hex(cnode)
             fparent1, fparent2 = repo.nullid, newfparent
-        else:
+        elif not repo.ui.configbool(b'experimental', b'copies.keep-old'):
             repo.ui.warn(
                 _(
                     b"warning: can't find ancestor for '%s' "
diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -943,6 +943,11 @@
 )
 coreconfigitem(
     b'experimental',
+    b'copies.keep-old',
+    default=False,
+)
+coreconfigitem(
+    b'experimental',
     b'crecordtest',
     default=None,
 )
diff --git a/mercurial/copies.py b/mercurial/copies.py
--- a/mercurial/copies.py
+++ b/mercurial/copies.py
@@ -53,10 +53,11 @@
     # between 5 and 6, so it includes all cases in its result.
     # Cases 1, 3, and 5 are then removed by _filter().
 
+    keep_old = src.repo().ui.configbool(b'experimental', b'copies.keep-old')
     for k, v in list(t.items()):
         if k == v:  # case 3
             del t[k]
-        elif v not in src:  # case 5
+        elif not keep_old and v not in src:  # case 5
             # remove copies from files that didn't exist
             del t[k]
         elif k not in dst:  # case 1
diff --git a/mercurial/metadata.py b/mercurial/metadata.py
--- a/mercurial/metadata.py
+++ b/mercurial/metadata.py
@@ -639,6 +639,9 @@
             p1copies[dst] = src
         elif src in p2 and p2[src].filenode() == srcnode:
             p2copies[dst] = src
+        else:
+            # This case should only happen with experimental.copies.keep-old
+            p1copies[dst] = src
     return p1copies, p2copies
 
 
diff --git a/tests/test-copies.t b/tests/test-copies.t
--- a/tests/test-copies.t
+++ b/tests/test-copies.t
@@ -754,6 +754,7 @@
   > [experimental]
   > evolution.createmarkers = True
   > evolution.allowunstable = True
+  > copies.keep-old = True
   > EOF
   $ newrepo
   $ echo a > a
@@ -769,11 +770,17 @@
   $ hg mv b c
   $ hg ci --amend -m "added c"
   1 new orphan changesets
+#if no-filelog no-compatibility
   $ hg rebase -s 'desc("modified b")' -d .
   rebasing 2:e3e0011b43ad "modified b" (changeset !)
   rebasing 2:2612b8963c3f "modified b" (no-changeset !)
+  merging c and b to c
+#else
+  $ hg rebase -s 'desc("modified b")' -d .
+  rebasing 2:2612b8963c3f "modified b"
   file 'b' was deleted in local [dest] but was modified in other [source].
   You can use (c)hanged version, leave (d)eleted, or leave (u)nresolved.
   What do you want to do? u
   unresolved conflicts (see 'hg resolve', then 'hg rebase --continue')
   [240]
+#endif