hmmmm.

A systematic policy of double crossing was given the UK government's official, if covert, sanction during the WWII. In 1941, MI5 set up a military counter espionage unit called The Twenty Committee, chaired by John Masterman. The naming of this unit clearly linked the double crosses of the Roman numerals for twenty (XX) with one of the unit's aims, which was to 'double cross' Germany by coercing German spies to become English double agents. The coercion was less than subtle; captured German agents were given an offer they couldn't refuse, that is, feed false information back to Germany or be shot.

During the Cold War, following the publishing of Masterman's The Double-Cross System in the War of 1939-1945, in 1972, the terms 'double cross' and 'double agent' became much more commonplace.

XX