British spies gave the Japanese information which helped them to knockout out Pearl Harbour and capture Singapore.
A BBC film by Paul Elston (BBC Two - The Fall of Singapore: The Great Betrayal) tells the story of how, for 19 years before the fall of Singapore to the Japanese, British officers were spying for Japan.
In addition, the Japanese got help from 'a mole who was a peer of the realm known to Churchill himself'.
In 1994 an ex-MI6 man, James Rusbridger, was found hanged at his house in Cornwall.
Much of the following is taken from 'The Intelligence Game' by James Rusbridger. (James Rusbridger)
In the 1930s and 1940s, MI6 failed to exploit the opposition to Hitler that existed within Germany.
In 1939, MI6 received the Oslo Report which contained details of Germany's secret weapons development. MI6 ignored the report.
In 1940, MI6 had an advance warning of the German plan to invade Holland and Belgium. But MI6 chose to ignore the warning.
In mid 1940 the Japanese discovered one of Britain's most important secrets: that Britain was not in a position to defend the Far East, including Singapore.
The Japanese had a copy of British War Cabinet minutes.
The minutes had been sent to Singapore as sea mail in a freighter that was seized by a German ship.
The Japanese saw the secret documents and proceeded to attack Pearl Harbour and Singapore, Malaysia and Indonesia.
Churchill created the Special Operations Executive.
On at least one occasion MI6 'infiltrated a double agent of its own into SOE who betrayed some 400 British and French agents to the Germans.'
(A retired Deputy Director of MI6 has stated that British intelligence knew about Argentina's intention to invade the Falklands. Argentina was invaded in 1982.)
More from Rusbridger:
Ex-SAS man planted bomb that killed 80 men, women and children.
Britain protected the terrorists found guilty of the Bologna Massacre
1988 Mossad plot to bomb Britain