diff --git a/debian/changelog b/debian/changelog
index a49cb51..8a77a29 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -2,6 +2,7 @@ ubuntu-dev-tools (0.129) UNRELEASED; urgency=low
 
   * syncpackage: Convert to new LP API, with --no-lp available for the old
     style of operation.
+  * syncpackage: Require -f/--force option to overwrite Ubuntu changes.
 
  -- Colin Watson <cjwatson@ubuntu.com>  Tue, 16 Aug 2011 16:40:22 +0100
 
diff --git a/doc/syncpackage.1 b/doc/syncpackage.1
index 979b748..67e7630 100644
--- a/doc/syncpackage.1
+++ b/doc/syncpackage.1
@@ -67,6 +67,9 @@ Do not sign the upload.
 \fB\-b\fI BUG\fR, \fB\-\-bug\fR=\fIBUG\fR
 Mark a Launchpad bug as being fixed by this upload.
 .TP
+\fB\-f\fR, \fB\-\-force\fR
+Force sync over the top of Ubuntu changes.
+.TP
 .B \-d \fIDEBIAN_MIRROR\fR, \fB\-\-debian\-mirror\fR=\fIDEBIAN_MIRROR\fR
 Use the specified mirror.
 Should be in the form \fBhttp://ftp.debian.org/debian\fR.
diff --git a/syncpackage b/syncpackage
index e5bbf99..7ccd6b4 100755
--- a/syncpackage
+++ b/syncpackage
@@ -116,7 +116,7 @@ def add_fixed_bugs(changes, bugs):
     return "\n".join(changes + [""])
 
 def sync_dsc(src_pkg, debian_dist, release, name, email, bugs, ubuntu_mirror,
-             keyid=None, simulate=False):
+             keyid=None, simulate=False, force=False):
     uploader = name + " <" + email + ">"
 
     src_pkg.pull_dsc()
@@ -142,6 +142,10 @@ def sync_dsc(src_pkg, debian_dist, release, name, email, bugs, ubuntu_mirror,
 
     cur_ver = ubuntu_ver.get_related_debian_version()
     if ubuntu_ver.is_modified_in_ubuntu():
+        if not force:
+            Logger.error('--force is required to discard Ubuntu changes.')
+            sys.exit(1)
+
         Logger.warn('Overwriting modified Ubuntu version %s, '
                     'setting current version to %s',
                     ubuntu_ver.full_version, cur_ver.full_version)
@@ -300,7 +304,8 @@ def fetch_source_pkg(package, dist, version, component, ubuntu_release, mirror):
     return DebianSourcePackage(package, version.full_version, component,
                                mirrors=[mirror])
 
-def copy(src_pkg, debian_dist, debian_version, release, simulate=False):
+def copy(src_pkg, debian_dist, debian_version, release, simulate=False,
+         force=False):
     debian = Distribution('debian')
     ubuntu = Distribution('ubuntu')
     debian_archive = debian.getArchive()
@@ -322,21 +327,25 @@ def copy(src_pkg, debian_dist, debian_version, release, simulate=False):
         ubuntu_version = ubuntu_archive.getSourcePackage(
             src_pkg, ubuntu_series, ubuntu_pocket).getVersion()
 
+        debian_version = Version(debian_version)
+        ubuntu_version = Version(ubuntu_version)
+
         Logger.normal('Source %s -> %s/%s: current version %s, new version %s',
                       src_pkg, ubuntu_series, ubuntu_pocket,
                       ubuntu_version, debian_version)
-        if Version(debian_version) <= Version(ubuntu_version):
+        if debian_version <= ubuntu_version:
             Logger.error('Debian version is <= Ubuntu version; nothing to do!')
             sys.exit(1)
+
+        if not force and ubuntu_version.is_modified_in_ubuntu():
+            Logger.error('--force is required to discard Ubuntu changes.')
+            sys.exit(1)
     except udtexceptions.PackageNotFoundException:
         Logger.normal('Source %s -> %s/%s: not in Ubuntu, new version %s',
                       src_pkg, ubuntu_series, ubuntu_pocket, debian_version)
     if simulate:
         return
 
-    if 'ubuntu' in ubuntu_version:
-        print "Ubuntu version had Ubuntu-specific modifications; check them " \
-              "carefully!"
     answer = YesNoQuestion().ask("Sync this package", "no")
     if answer != "yes":
         return
@@ -344,7 +353,7 @@ def copy(src_pkg, debian_dist, debian_version, release, simulate=False):
     try:
         ubuntu_archive.copyPackage(
             source_name=src_pkg,
-            version=debian_version,
+            version=str(debian_version),
             from_archive=debian_archive,
             to_series=ubuntu_series,
             to_pocket=ubuntu_pocket,
@@ -405,6 +414,9 @@ def main():
                       dest="bugs", action="append", default=list(),
                       help="Mark Launchpad bug BUG as being fixed by this "
                            "upload.")
+    parser.add_option("-f", "--force",
+                      dest="force", action="store_true", default=False,
+                      help="Force sync over the top of Ubuntu changes.")
     parser.add_option('-D', '--debian-mirror', metavar='DEBIAN_MIRROR',
                       dest='debian_mirror',
                       help='Preferred Debian mirror '
@@ -473,7 +485,7 @@ def main():
             sys.exit(1)
 
         copy(args[0], options.dist, options.debversion, options.release,
-             options.simulate)
+             options.simulate, options.force)
     else:
         Launchpad.login_anonymously()
         if options.release is None:
@@ -488,7 +500,7 @@ def main():
 
         sync_dsc(src_pkg, options.dist, options.release, options.uploader_name,
                  options.uploader_email, options.bugs, options.ubuntu_mirror,
-                 options.keyid, options.simulate)
+                 options.keyid, options.simulate, options.force)
 
 if __name__ == "__main__":
     main()