It is unusual for a historian to play a role in great events himself, but Asa Briggs has both interpreted history to a mass audience and played no small part in the making of it. There aren't many people still around who have checked proofs of Churchill's books for historical accuracy, worked alongside the computer pioneer Alan Turing, and discussed revolution with Zhou Enlai and atomic science with J Robert Oppenheimer.

Now 92, Briggs  Lord Briggs of Lewes, to give this socialist peer his correct baronial title  has published his second volume of autobiography after the acclaimed Secret Days  the long-delayed revelation of his wartime role as part of Turing's code-breaking team at Bletchley Park, where Nazi Germany's Enigma ciphers were cracked and their entire military signals traffic laid open to allied eyes  including those of young Briggs, recruited by talent spotters at Cambridge University, from where he had just graduated.

Special Relationships looks back in affection at other areas of Briggs's varied life, from his childhood in Keighley between the wars to his later globe-trotting as an academic historian and educational expert. (He was instrumental in setting up both Sussex University, of which he became vice-chancellor, and the Open University.) A Labour man born and bred, Briggs regards Clement Attlee  whom he accompanied to Turkey on a 1950s cruise  as Britain's greatest prime minister; he regrets that a fellow Sussex nonagenarian friend, Denis Healey, never held the top job. Two other prime ministers, Harold Macmillan and Jim Callaghan, were also Sussex neighbours counted by Briggs as friends. He is scathing, by comparison, about today's generation of political leaders, deploring the ignorance and lack of interest in Britain's history displayed by Cameron and Clegg.

Building on the grand tradition of GM Trevelyan's English Social History, Briggs's work on the Victorians  his grandparents' generation  is a celebration of the entrepreneurial achievements of 19th-century Britain and a tribute to the working people who constructed the cities, fuelled the factories and ran the railways. His most highly regarded work includes The Age of Improvement, and the trilogy Victorian People, Victorian Cities and Victorian Things. Never a Marxist, Briggs perhaps opened the gates for a more radical reappraisal of the history of the English working class in the hands of EP Thompson and Christopher Hill.

What is striking in his autobiography is the breadth of his interests and activities, which include collecting Maoist ceramics. Not for Briggs the chasm between the "two cultures" identified by CP Snow, one of his Cambridge mentors. But despite his multifarious public activities since the war, it is his secret role during the conflict of which he is most proud. Briggs feels it unjust that Cambridge University is more notorious today for the spies it bred in the 1930s (Philby, Burgess, Maclean, Blunt and so on) than it is celebrated for the codebreakers and cryptographers it produced a generation later, whose minds hatched what Churchill called "the golden eggs" of war-winning intelligence.

Briggs reached Cambridge in December 1937 as a precocious 16-year-old, fresh out of Keighley grammar school. "I went up to Sidney Sussex College and the don who interviewed me said: 'You are just a baby, Briggs, but war is coming and you should take your degree before you put on a uniform.'" By the time the war arrived, he had already been recruited into the secret world, in the way the intelligence community used to refresh its ranks before the days of open job ads and interview boards.

"Gordon Welchman, who organised Bletchley, picked three people from my college," Briggs recalls. "I was originally intended for radar research  which was still a secret project. CP Snow was recruiting for the government. He sent for me. Although it was a darkened room in December, I could just make him out in the gloom and thought him the ugliest man I had ever seen. I was then told that since radar was now up and running they had no further need for historians to convert into scientists and I should go into the army after all." Briggs joined the intelligence corps, and in 1943 the call came from Bletchley. The team he joined worked mainly on signals traffic in the Mediterranean theatre, using the "Bombes"  the proto-computers devised by Turing  which were a mirror image of Germany's Enigma encryption machines, whose messages they were reading. Bletchley was the 1940s equivalent of a huge computer-hacking plant.

In the run-up to D-day Briggs was involved in the vast deceptions recently detailed in books by Ben Macintyre. In Operation Fortitude, the Germans were duped into thinking that the invasion of Europe would come anywhere but its real target  Normandy. "And I knew all about Operation Mincemeat, in which a tramp's corpse dressed in an officer's uniform and carrying totally misleading faked 'secret' papers was dumped into the sea off Spain so it would fall into German hands," Briggs writes.

The socialist in Briggs  he turned down an offer from Hugh Dalton to stand for a safe Labour seat in the 1945 landslide, preferring to resume his academic career  is still stirred by the memory of the equality and camaraderie among the Bletchley boffins. "There was no question of rank. Everyone from Turing down to the women who did the menial typing and catering were treated equally. Except that we all deferred to Turing, as it was generally realised that he was a genius."

A fellowship lured Briggs to Oxford's Worcester College, where he helped Churchill with his monumental History of the English Speaking Peoples. ("It won him the Nobel prize for literature, not that I can claim the credit for that.") He takes perverse pride in a rare failure: an article on army slang that he sent to George Orwell at Tribune magazine was turned down flat.

Briggs's study is adorned with his collection of Chinese revolutionary pottery  the fruit of his journeys to the country with the BBC. I counted at least 15 statuettes of the Great Helmsman himself. But wasn't Mao a monster? "He was. But you cannot escape his influence, just as you cannot avoid that of Stalin in Russia or Hitler in Germany. I don't believe in history's final judgments. We will still be arguing about these men at the end of time."

 Nigel Jones's Tower: An Epic History of the Tower of London is published by Windmill.