The Bristol Blenheim was used by both Bomber and Fighter Commands. Some two hundred Mk I bombers were modified into Mk IF long-range fighters with 600 (Auxiliary Air Force) Squadron, based at Hendon, the first squadron to take delivery in September 1938. By 1939, at least seven squadrons were operating these twin-engined fighters and within a few months, some sixty squadrons had experience of the type. The Mk IF proved to be slower and less nimble than expected, and by June 1940, daylight Blenheim losses were to cause concern for Fighter Command. It was decided that the Mk IF would be relegated mainly to night fighter duties where No. 23 Squadron RAF, which had already operated the type under nighttime conditions, had better success.