Index: mercurial/bundle2.py
===================================================================
--- mercurial/bundle2.py
+++ mercurial/bundle2.py
@@ -299,7 +299,7 @@
     * a way to construct a bundle response when applicable.
     """
 
-    def __init__(self, repo, transactiongetter, captureoutput=True, source=''):
+    def __init__(self, repo, transactiongetter, captureoutput=True, source='', force=False):
         self.repo = repo
         self.ui = repo.ui
         self.records = unbundlerecords()
@@ -310,6 +310,7 @@
         # carries value that can modify part behavior
         self.modes = {}
         self.source = source
+        self.force = force
 
     def gettransaction(self):
         transaction = self._gettransaction()
@@ -2135,17 +2136,23 @@
     if bookmarksmode == 'apply':
         tr = op.gettransaction()
         bookstore = op.repo._bookmarks
+        allhooks = []
+        for book, node in changes:
+            old_node = bookstore.get(book, '')
+            if not op.force and old_node != '' and node is not None:
+                if not bookmarks.validdest(op.repo, op.repo[old_node], op.repo[node]):
+                    raise error.Abort(_('push rejected: bookmark "%s" has changed') % book,
+                                      hint=_("run 'hg pull', resolve conflicts, and push again"))
+            hookargs = tr.hookargs.copy()
+            hookargs['pushkeycompat'] = '1'
+            hookargs['namespace'] = 'bookmarks'
+            hookargs['key'] = book
+            hookargs['old'] = nodemod.hex(old_node)
+            hookargs['new'] = nodemod.hex(node if node is not None else '')
+            hookargs['force'] = op.force
+            allhooks.append(hookargs)
+
         if pushkeycompat:
-            allhooks = []
-            for book, node in changes:
-                hookargs = tr.hookargs.copy()
-                hookargs['pushkeycompat'] = '1'
-                hookargs['namespace'] = 'bookmarks'
-                hookargs['key'] = book
-                hookargs['old'] = nodemod.hex(bookstore.get(book, ''))
-                hookargs['new'] = nodemod.hex(node if node is not None else '')
-                allhooks.append(hookargs)
-
             for hookargs in allhooks:
                 op.repo.hook('prepushkey', throw=True,
                              **pycompat.strkwargs(hookargs))
Index: mercurial/exchange.py
===================================================================
--- mercurial/exchange.py
+++ mercurial/exchange.py
@@ -1159,6 +1159,7 @@
                     'bundle': stream,
                     'heads': ['force'],
                     'url': pushop.remote.url(),
+                    'force': pushop.force,
                 }).result()
         except error.BundleValueError as exc:
             raise error.Abort(_('missing support for %s') % exc)
@@ -2375,7 +2376,7 @@
         raise error.PushRaced('repository changed while %s - '
                               'please try again' % context)
 
-def unbundle(repo, cg, heads, source, url):
+def unbundle(repo, cg, heads, source, url, force=None):
     """Apply a bundle to a repo.
 
     this function makes sure the repo is locked during the application and have
@@ -2424,7 +2425,8 @@
 
                 op = bundle2.bundleoperation(repo, gettransaction,
                                              captureoutput=captureoutput,
-                                             source='push')
+                                             source='push',
+                                             force=force)
                 try:
                     op = bundle2.processbundle(repo, cg, op=op)
                 finally:
Index: mercurial/localrepo.py
===================================================================
--- mercurial/localrepo.py
+++ mercurial/localrepo.py
@@ -306,14 +306,14 @@
         raise error.Abort(_('cannot perform stream clone against local '
                             'peer'))
 
-    def unbundle(self, bundle, heads, url):
+    def unbundle(self, bundle, heads, url, force=None):
         """apply a bundle on a repo
 
         This function handles the repo locking itself."""
         try:
             try:
                 bundle = exchange.readbundle(self.ui, bundle, None)
-                ret = exchange.unbundle(self._repo, bundle, heads, 'push', url)
+                ret = exchange.unbundle(self._repo, bundle, heads, 'push', url, force=force)
                 if util.safehasattr(ret, 'getchunks'):
                     # This is a bundle20 object, turn it into an unbundler.
                     # This little dance should be dropped eventually when the
Index: mercurial/repository.py
===================================================================
--- mercurial/repository.py
+++ mercurial/repository.py
@@ -191,7 +191,7 @@
         Successful result should be a generator of data chunks.
         """
 
-    def unbundle(bundle, heads, url):
+    def unbundle(bundle, heads, url, force=None):
         """Transfer repository data to the peer.
 
         This is how the bulk of data during a push is transferred.
Index: mercurial/wireprotov1peer.py
===================================================================
--- mercurial/wireprotov1peer.py
+++ mercurial/wireprotov1peer.py
@@ -445,7 +445,7 @@
         else:
             return changegroupmod.cg1unpacker(f, 'UN')
 
-    def unbundle(self, bundle, heads, url):
+    def unbundle(self, bundle, heads, url, force=None):
         '''Send cg (a readable file-like object representing the
         changegroup to push, typically a chunkbuffer object) to the
         remote server as a bundle.
Index: tests/test-bookmarks-conflict.t
===================================================================
--- /dev/null
+++ tests/test-bookmarks-conflict.t
@@ -0,0 +1,91 @@
+initialize
+  $ make_changes() {
+  >     d=`pwd`
+  >     [ ! -z $1 ] && cd $1
+  >     echo "test `basename \`pwd\``" >> test
+  >     hg commit -Am"${2:-test}"
+  >     r=$?
+  >     cd $d
+  >     return $r
+  > }
+  $ ls -1a
+  .
+  ..
+  $ hg init a
+  $ cd a
+  $ echo 'test' > test; hg commit -Am'test'
+  adding test
+  $ hg book @
+
+clone to b
+
+  $ mkdir ../b
+  $ cd ../b
+  $ hg clone ../a .
+  updating to bookmark @
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ make_changes
+  $ hg book bk_b
+
+clone to c
+  $ mkdir ../c
+  $ cd ../c
+  $ hg clone ../a .
+  updating to bookmark @
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ make_changes
+  $ hg book bk_c
+
+push from b
+  $ cd ../b
+  $ hg push -B .
+  pushing to $TESTTMP/a
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+  updating bookmark @
+  exporting bookmark bk_b
+  $ hg -R ../a id -r @
+  e11a942451be tip @/bk_b
+
+push from c
+  $ cd ../c
+  $ hg push -B .
+  pushing to $TESTTMP/a
+  searching for changes
+  remote has heads on branch 'default' that are not known locally: e11a942451be
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files (+1 heads)
+  exporting bookmark bk_c
+  $ hg push -B @
+  pushing to $TESTTMP/a
+  searching for changes
+  no changes found
+  abort: push rejected: bookmark "@" has changed
+  (run 'hg pull', resolve conflicts, and push again)
+  [255]
+  $ hg -R ../a log -G -T '{rev} {bookmarks}'
+  o  2 bk_c
+  |
+  | o  1 @ bk_b
+  |/
+  @  0
+  
+
+  $ hg push -B @ --force
+  pushing to $TESTTMP/a
+  searching for changes
+  no changes found
+  updating bookmark @
+  [1]
+  $ hg -R ../a log -G -T '{rev} {bookmarks}'
+  o  2 @ bk_c
+  |
+  | o  1 bk_b
+  |/
+  @  0
+