diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -2135,7 +2135,8 @@ def undo(ui, repo):
     """
     repo.undo()
 
-def update(ui, repo, node=None, merge=False, clean=False, branch=None):
+def update(ui, repo, node=None, merge=False, clean=False, force=None,
+           branch=None):
     """update or merge working directory
 
     Update the working directory to the specified revision.
@@ -2172,7 +2173,7 @@ def update(ui, repo, node=None, merge=Fa
             return 1
     else:
         node = node and repo.lookup(node) or repo.changelog.tip()
-    return repo.update(node, allow=merge, force=clean)
+    return repo.update(node, allow=merge, force=clean, forcemerge=force)
 
 def verify(ui, repo):
     """verify the integrity of the repository
@@ -2418,8 +2419,9 @@ table = {
         (update,
          [('b', 'branch', "", _('checkout the head of a specific branch')),
           ('m', 'merge', None, _('allow merging of branches')),
-          ('C', 'clean', None, _('overwrite locally modified files'))],
-         _('hg update [-b TAG] [-m] [-C] [REV]')),
+          ('C', 'clean', None, _('overwrite locally modified files')),
+          ('f', 'force', None, _('force a merge with outstanding changes'))],
+         _('hg update [-b TAG] [-m] [-C] [-f] [REV]')),
     "verify": (verify, [], _('hg verify')),
     "version": (show_version, [], _('hg version')),
 }
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -1358,7 +1358,7 @@ class localrepository(object):
         return
 
     def update(self, node, allow=False, force=False, choose=None,
-               moddirstate=True):
+               moddirstate=True, forcemerge=False):
         pl = self.dirstate.parents()
         if not force and pl[1] != nullid:
             self.ui.warn(_("aborting: outstanding uncommitted merges\n"))
@@ -1378,6 +1378,18 @@ class localrepository(object):
 
         (c, a, d, u) = self.changes()
 
+        if allow and not forcemerge:
+            if c or a or d:
+                raise util.Abort(_("outstanding uncommited changes"))
+        if not forcemerge and not force:
+            for f in u:
+                if f in m2:
+                     t1 = self.wread(f)
+                     t2 = self.file(f).read(m2[f])
+                     if cmp(t1, t2) != 0:
+                        raise util.Abort(_("'%s' already exists in the working"
+                                           " dir and differs from remote") % f)
+
         # is this a jump, or a merge?  i.e. is there a linear path
         # from p1 to p2?
         linear_path = (pa == p1 or pa == p2)
diff --git a/tests/test-merge1 b/tests/test-merge1
--- a/tests/test-merge1
+++ b/tests/test-merge1
@@ -40,8 +40,10 @@ echo This is file c1 > c
 hg add c
 hg commit -m "commit #2" -d "0 0"
 echo This is file b2 > b
+echo %% merge should fail
+env HGMERGE=../merge hg update -m 1
 echo %% merge of b expected
-env HGMERGE=../merge hg update -m 1
+env HGMERGE=../merge hg update -f -m 1
 cd ..; /bin/rm -rf t
 echo %%
 
@@ -65,8 +67,10 @@ echo 'Contents of b should be "this is f
 cat b
 
 echo This is file b22 > b
+echo %% merge fails
+env HGMERGE=../merge hg update -m 2
 echo %% merge expected!
-env HGMERGE=../merge hg update -m 2
+env HGMERGE=../merge hg update -f -m 2
 cd ..; /bin/rm -rf t
 
 mkdir t
@@ -85,6 +89,8 @@ echo This is file c1 > c
 hg add c
 hg commit -m "commit #3" -d "0 0"
 echo This is file b33 > b
-echo %% merge of b expected
+echo %% merge of b should fail
 env HGMERGE=../merge hg update -m 2
+echo %% merge of b expected
+env HGMERGE=../merge hg update -f -m 2
 cd ..; /bin/rm -rf t
diff --git a/tests/test-merge1.out b/tests/test-merge1.out
--- a/tests/test-merge1.out
+++ b/tests/test-merge1.out
@@ -1,13 +1,19 @@
 %% no merges expected
+%% merge should fail
+abort: 'b' already exists in the working dir and differs from remote
 %% merge of b expected
 merging for b
 merging b
 %%
 Contents of b should be "this is file b1"
 This is file b1
+%% merge fails
+abort: outstanding uncommited changes
 %% merge expected!
 merging for b
 merging b
+%% merge of b should fail
+abort: outstanding uncommited changes
 %% merge of b expected
 merging for b
 merging b
diff --git a/tests/test-up-local-change b/tests/test-up-local-change
--- a/tests/test-up-local-change
+++ b/tests/test-up-local-change
@@ -25,7 +25,8 @@ cd ../r2
 hg -q pull ../r1
 hg status
 hg --debug up
-hg --debug up -m
+hg --debug up -m || echo failed
+hg --debug up -f -m
 hg parents
 hg -v history
 hg diff | sed -e "s/\(+++ [a-zA-Z0-9_/.-]*\).*/\1/" \
diff --git a/tests/test-up-local-change.out b/tests/test-up-local-change.out
--- a/tests/test-up-local-change.out
+++ b/tests/test-up-local-change.out
@@ -16,6 +16,8 @@ getting b
 merging a
 resolving a
 file a: my b789fdd96dc2 other d730145abbf9 ancestor b789fdd96dc2
+abort: outstanding uncommited changes
+failed
 resolving manifests
  force None allow 1 moddirstate True linear True
  ancestor 1165e8bd193e local 1165e8bd193e remote 1165e8bd193e