diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -608,13 +608,11 @@
                     storevfs,
                     requirements,
                 )
-            elif ui.configbool(
-                b'experimental', b'sharesafe-warn-outdated-shares'
-            ):
-                ui.warn(
+            else:
+                raise error.Abort(
                     _(
-                        b'warning: source repository supports share-safe functionality.'
-                        b' Reshare to upgrade.\n'
+                        b'version mismatch: source uses share-safe'
+                        b' functionality while the current share does not'
                     )
                 )
 
diff --git a/tests/test-share-safe.t b/tests/test-share-safe.t
--- a/tests/test-share-safe.t
+++ b/tests/test-share-safe.t
@@ -392,20 +392,11 @@
   o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
   
 
-Make sure existing shares still works
-
-  $ hg log -GT "{node}: {desc}\n" -R ../nss-share --config experimental.sharesafe-warn-outdated-shares=false
-  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
-  |
-  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
-  
+Make sure existing shares dont work with default config
 
   $ hg log -GT "{node}: {desc}\n" -R ../nss-share
-  warning: source repository supports share-safe functionality. Reshare to upgrade.
-  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
-  |
-  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
-  
+  abort: version mismatch: source uses share-safe functionality while the current share does not
+  [255]
 
 
 Create a safe share from upgrade one
@@ -538,11 +529,8 @@
   sparserevlog
   store
   $ hg log -GT "{node}: {desc}\n" -R ../nss-share
-  warning: source repository supports share-safe functionality. Reshare to upgrade.
-  @  f63db81e6dde1d9c78814167f77fb1fb49283f4f: added bar
-  |
-  o  f3ba8b99bb6f897c87bbc1c07b75c6ddf43a4f77: added foo
-  
+  abort: version mismatch: source uses share-safe functionality while the current share does not
+  [255]
 
 Check that if lock is taken, upgrade fails but read operation are successful
   $ touch ../nss-share/.hg/wlock