Sixsmith told he will lose any payments if he goes public

Martin Sixsmith, the former head of communications at the Department of Transport, has been told he will not only lose his £180,000 pay-off if talks publicly about his time there, but any speaker's fees, TV salaries and payments for articles.

Mr Sixsmith was informed that if he revealed details, the Government would "enforce" an obligation of silence by "instituting proceedings to recover the monies paid, together with an account of profits resulting from a breach of those obligations".

A leaked letter from Mr Sixsmith to the most senior civil servant in the office of John Prescott, the Deputy Prime Minister, which has been circulated around Whitehall, has been seen by The Independent.

It details the "gagging" demands that the Government has imposed. The letter quotes correspondence from the Government, which confirms that it is "delaying the payment (of your compensation award)" until it has an assurance he will remain silent and requires "a further express assurance" that he will not produce "books, films, newspaper articles, television or any other media drawing on your experience in Government".

Mr Sixsmith asks in the letter for a commitment that he will not be penalised if he gives evidence to the Wicks Committee on standards into the role of special advisers.