The light armament was seldom able to deter fighter opposition. Squadrons were forced to use several different improvisations in an attempt to provide better defensive armament, until officially sanctioned modifications were able to be introduced in early 1940. The Blenheim also proved to be vulnerable to anti-aircraft artillery, especially around the rear fuselage. Flexible, self-sealing liners had been fitted to the fuel tanks but they were still not fully protected against the MG FF cannon carried by the "Luftwaffe"s Bf 109 and Bf 110 fighters.