Stephen Byers' former media chief is being pushed into signing a new "gagging clause" before receiving a pay-off from the government, according to a Sunday newspaper.

Martin Sixsmith told friends he had been subjected to intimidation and attempts to make his life a misery, according to the Independent on Sunday.

The clause reportedly stops Mr Sixsmith from speaking about his time working for the government.

He has already agreed not to talk about his pay-off - reported to be £180,000.

Mr Sixsmith was asked to resign following the row over an email sent by Mr Byer's spin doctor Jo Moore that suggested 11 September was a "good day" to bury bad news.

He agreed not to reveal details of his pay-off and the negotiations surrounding them in a legally-binding severance contract signed on 25 April.

'Draconian'

But on 31 May, when Mr Sixsmith was due to receive the money, government officials and lawyers reportedly told him he must sign a new clause preventing him from talking about his five years in Whitehall.

His legal team is said to believe the "very draconian" new clause is unconstitutional and breaches the European Convention on Human Rights.

A legal challenge could see the prime minister's director of communications Alistair Campbell and other senior civil servants asked to give evidence.

The Observer newspaper suggests the leaking of e-mails asking about the political affiliations of key members of the Paddington Survivors Group has made the government "paranoid" that more secrets may be revealed.

A source involved in the negotiations also claimed the government is trying to smear the former BBC journalist.

His former colleagues had received calls asking if he had tried to undermine ex-director general Lord Birt - now in charge of "blue-skies thinking" on transport for the prime minister, the source told the newspaper.