diff --git a/build.py b/build.py
index 255c8d1da10..e1201f6842b 100755
--- a/build.py
+++ b/build.py
@@ -89,6 +89,7 @@ def java_omero(args):
     command = [ find_java() ]
     p = os.path.join( os.path.curdir, "lib", "log4j-build.xml")
     command.append("-Dlog4j.configuration=%s" % p)
+    command.append("-Dbuild.py=true")
     command.extend( calculate_memory_args() )
     command.extend(["omero"])
     command.extend(choose_omero_version())
diff --git a/components/antlib/resources/global.xml b/components/antlib/resources/global.xml
index 4d1f80817b4..b2222079ddd 100644
--- a/components/antlib/resources/global.xml
+++ b/components/antlib/resources/global.xml
@@ -214,6 +214,19 @@
         <taskdef resource="net/sf/antcontrib/antlib.xml"
                  classpath="${lib.dir}/repository/ant-contrib-1.0b3.jar"/>
 
+        <!-- prevent direct use of ant -->
+        <if><not><isset property="build.py"/></not>
+            <then>
+                <fail> ERROR!
+
+========================================================
+Please use ./build.py which uses the ant version bundled
+with OMERO.
+========================================================
+                </fail>
+            </then>
+        </if>
+
         <!-- Define properties :
                 The user's environment (env) overrides all other
                 variables, then comes the user defined