British police have discovered forged documents were planted in the National Archives alleging top Nazi Heinrich Himmler was murdered on Winston Churchill's orders, the Public Record Office said on Saturday.

The investigation identified 29 forgeries that had been slipped into 12 files after 2000. The office said it would introduce improved security measures, including cameras in research areas, to ensure there is no repeat of the forgery.

Forensic examination revealed letterheads on documents purported to be written by Bernard Bracken, minister of information for wartime prime minister Churchill, in 1945 were made with a laser printer.