diff --git a/mercurial/context.py b/mercurial/context.py
--- a/mercurial/context.py
+++ b/mercurial/context.py
@@ -396,7 +396,7 @@ class changectx(basectx):
 
     def ancestor(self, c2):
         """
-        return the ancestor context of self and c2
+        return the "best" ancestor context of self and c2
         """
         # deal with workingctxs
         n2 = c2._node
@@ -408,10 +408,19 @@ class changectx(basectx):
         elif len(cahs) == 1:
             anc = cahs[0]
         else:
-            anc = self._repo.changelog.ancestor(self._node, n2)
+            for r in self._repo.ui.configlist('merge', 'preferancestor'):
+                ctx = changectx(self._repo, r)
+                anc = ctx.node()
+                if anc in cahs:
+                    break
+            else:
+                anc = self._repo.changelog.ancestor(self._node, n2)
             self._repo.ui.status(
                 (_("note: using %s as ancestor of %s and %s\n") %
-                 (short(anc), short(self._node), short(n2))))
+                 (short(anc), short(self._node), short(n2))) +
+                ''.join(_("      alternatively, use --config "
+                          "merge.preferancestor=%s\n") %
+                        short(n) for n in sorted(cahs) if n != anc))
         return changectx(self._repo, anc)
 
     def descendant(self, other):
diff --git a/tests/test-merge-criss-cross.t b/tests/test-merge-criss-cross.t
--- a/tests/test-merge-criss-cross.t
+++ b/tests/test-merge-criss-cross.t
@@ -25,6 +25,7 @@ Criss cross merging
 
   $ hg up -r3
   note: using 0f6b37dbe527 as ancestor of adfe50279922 and cf89f02107e5
+        alternatively, use --config merge.preferancestor=40663881a6dd
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ echo '6 second change' > f2
   $ hg ci -m '6 second change f2'
@@ -75,6 +76,7 @@ Criss cross merging
 
   $ hg merge -v --debug --tool internal:dump 5
   note: using 0f6b37dbe527 as ancestor of 3b08d01b0ab5 and adfe50279922
+        alternatively, use --config merge.preferancestor=40663881a6dd
     searching for copies back to rev 3
   resolving manifests
    branchmerge: True, force: False, partial: False
@@ -111,4 +113,14 @@ Criss cross merging
   ==> f2.other <==
   2 first change
 
+  $ hg up -qC .
+  $ hg merge -v --tool internal:dump 5 --config merge.preferancestor="null 40663881 3b08d"
+  note: using 40663881a6dd as ancestor of 3b08d01b0ab5 and adfe50279922
+        alternatively, use --config merge.preferancestor=0f6b37dbe527
+  resolving manifests
+  merging f1
+  0 files updated, 0 files merged, 0 files removed, 1 files unresolved
+  use 'hg resolve' to retry unresolved file merges or 'hg update -C .' to abandon
+  [1]
+
   $ cd ..