The Vassall Tribunal was a public inquiry undertaken in 1963 by the British government in the wake of the John Vassall affair. Vassall, a civil servant working in the Admiralty, had been revealed the previous year to be a Soviet spy, and considerable criticism had been leveled at the security arrangements that were in place. The tribunal was established to investigate these claims, and determine whether any blame could be laid on officials or ministers.

At first, the inquiry was to be conducted by three senior civil servants — the Permanent Under-Secretary of the Treasury, the Treasury Solicitor, and the Second Secretary at the Treasury. Before it could begin, however, letters were discovered in Vassall's possession from Tam Galbraith, who had been a Civil Lord of the Admiralty. It was thought odd that a minister would communicate by post with an official of his own department, and there was considerable speculation of impropriety in the press. Given Vassall's known homosexuality, rumours began to circulate that Vassall and Galbraith were involved with each other, and that Galbraith might have shielded Vassall from discovery. The committee of civil servants investigated the correspondence and declared it innocent, but this verdict was not universally accepted. Eventually, the Prime Minister was compelled to open a wider inquiry, conducted by three jurists. Eventually, the inquiry determined that Vassall had not been helped or favoured by any of his seniors.

The inquiry was controversial in some quarters for compelling journalists to reveal the sources behind their allegations, and for having two journalists (including Brendan Mulholland of the Daily Mail and Reg Foster of the Daily Sketch) who refused prosecuted and jailed for contempt of court.

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