EP Thompson, one of the most influential historians of the 20th century, wrote an impassioned denunciation of the leadership of the British Communist party at the height of the cold war, but few outside MI5 knew anything about it after the agency intercepted the letter.

The party’s leaders, Thompson said, were despotic and untrustworthy, and would sweep away long-cherished political freedoms if they ever achieved power.

Thompson, himself a Communist party member at the time, wrote the letter in 1956, shortly after the Soviet leader Nikita Khrushchev gave a speech in which, for the first time, he criticised Joseph Stalin, who had died three years earlier.

A copy of the letter lies within MI5’s files on Thompson, five of which have been declassified and transferred to the National Archives at Kew, in south-west London.

It shows Thompson’s clear dismay that leading communists in Britain may have been aware of Stalin’s crimes, yet had been responsible for misleading rank-and-file members and for spending two decades spreading “uncritical and often hopelessly inaccurate (our opponents would say mendacious) propaganda.” ...