Now it can be told, as the old Fleet Street saying has it. Donald Bruce, parliamentary private secretary to Nye Bevan in the Attlee government, who died during the election campaign aged 92, took to his grave the secret of his hero's sensational conversion to the wisdom of Britain's nuclear deterrent.
Bevan shocked his friends and gratified his enemies in the Labour establishment in 1957, when he warned the annual party conference that nuclear unilateral disarmament would send a British foreign secretary "naked into the conference chamber", a phrase that has entered the political lexicon.
His intervention, which swung delegates and the union block vote in favour of the platform, was regarded by the left as an act of treachery, an unwarranted slur on socialism and an attack on the Soviet Union to boot.
In fact, Bevan was acting with the full knowledge and endorsement of the Kremlin, almost, indeed, on its instructions. During his trip to Moscow some months earlier, after taking over the foreign portfolio, the Russians had strongly conveyed to Bevan their desire that Britain should keep the bomb as a bargaining counter in the tortuous game of realpolitik with the US.
That, at least, was the firm view of Donald Bruce. As an adviser to Bevan, though no longer MP for Portsmouth North, he was brought into the secret. Decades later, he would regale friends with it after a few glasses of wine in the Strangers' Bar at Westminster. For some reason (perhaps an old-fashioned respect for the Daily Mirror of the war years), he chose to tell me the story, though he did not want it to go into print with his name attached.
Even 50 years afterwards, he was reluctant to break the bonds of reticence that so characterised old soldiers like him, who came to Westminster in 1945 still in their army uniform. Michael Foot touches on the issue in his monumental biography of Bevan, only to dismiss it. In the immediate aftermath of the Brighton conference, he writes, some had argued that Nye's conversations with Khrushchev had tipped the balance. The Russian leader had stressed how important it was to keep the bomb and "how contemptuous he would be if we abandoned it".
Foot scorns "the paradox that Mr Khrushchev had backed Britain having the bomb because he wanted Britain to be suitably equipped to deter Mr Khrush- chev", while conceding that the visit to Moscow had influenced Bevan.
Donald Bruce was adamant that it had actually made up his mind, which explains his subsequent conduct far more rationally than any supposed desire to curry favour with Hugh Gaitskell.
Almost half a century later, the bomb still stirs political feelings.
During the election campaign, Blair - a member of Parliamentary CND in 1986 - spoke passionately about the need for a British nuclear deterrent (even he is wise enough not to call it "independent"), while leaving open the question of what will follow Trident.
But then, he takes his orders from Washington, not Moscow. So, it was no surprise to read that Blair has actually decided to go ahead with a £10bn replacement. This should be one of the first targets for Labour's awkward squad when parliament gets under way again.
Bruce's death removed virtually the last living link with the 1945 Labour government. He was belatedly given a peerage by Harold Wilson in 1974, and in the Lords was a relentless critic of the Common Market, as it then was.
Awkwardness was nothing new. On one occasion, when he attacked Labour's foreign policy, Clem Attlee asked: "Feel better now, Donald?"
"That was the attitude towards dissent in those days," he once recollected ruefully. "Now, to dissent is almost to be a heretic. Yet it is only by dissent that there is progress. That is the dialogue of life."
His views on Tony Blair may be gleaned from that aside, though he did occasionally use language more suited to the barrack room.