Gordon Brown on visit to Liverpool (320)

Mr Field, a long-standing critic of Mr Brown – he led the Labour revolt against the scrapping of the 10p tax rate – could not be contacted, but is understood to have also demanded nomination papers.

It is the second time Mr Howarth has broken ranks to pile pressure on the Prime Minister, having urged fellow Labour MPs to consider ditching him in the wake of July’s Glasgow by-election.

The former Home Office minister is an ally of Justice Secretary Jack Straw, but insisted he was not acting to try to propel any particular candidate into No.10.

Mr Howarth added: “I hope as many people as possible will come forward, so that we have a real choice of leaders.”

Simultaneously, Mr Howarth was among 12 Labour MPs, including six former ministers, who released a magazine article warning that Labour faced a “hammer blow” if it failed to change direction.

Although the demand for a contest was a blow to Mr Brown’s authority, there were suspicions that Downing Street leaked the information – late on a Friday – to try to minimise the impact.

The Prime Minister’s allies may have feared that more MPs would come forward and that the damage would have been even worse on the eve of Labour’s conference.

Even so, there were suggestions that Labour would now have to issue nomination papers to all its MPs. It is party policy to do so every autumn, but that has not happened since 1998.