They also show that Britain, and many other European states, had known about Enigma since the early 1920s and that British intelligence had the full specification of early versions of the machine in 1925.
The UK Government has released secret files about how it managed to break the Enigma machine code system, used by the German military in the Second World War.
The files, held at the Public Record Office in London, reveal that Britain only managed to crack the code with help from Polish intelligence.
They also show that Britain, and many other European states, had known about Enigma since the early 1920s and that British intelligence had the full specification of early versions of the machine in 1925.
Post-war histories of how the Enigma system was cracked have tended to focus on the work of wartime staff at Britain's secret eavesdropping headquarters at Bletchley Park.
The success it achieved, in reading secret German military traffic, shortened the Second World War and was hailed as a triumph of British ingenuity in the face of an astute and devious adversary.
But the files now released paint a picture more complex.
Polish photographs
It turns out that British Intelligence first saw the full specification of early Enigma machines, with their cogs, wheels and wiring, in 1925, when the design was lodged at the Patent Office in London.
Enigma was first used in a war theatre by the Fascist forces in Spain and Italy, in the Spanish civil war in 1937.
British code-breakers had some success with the increasingly difficult and seemingly random code systems that the Germans introduced.
But when it came to the crunch, in 1939, Britain was given crucial help by Polish Intelligence, which supplied parts of a captured 1938 Enigma machine and photographs of its encoding systems and provided vital details of the wiring system.
Only when that had been achieved could the British put to work their secret army of clerks, methodically sifting and laying bare a mass of German secrets.