Gordon Brown: Said to be no chance of him standing down
Prime Minister Gordon Brown's allies have gone on the offensive about his future after recent resignations.
Sources say there is "no chance" of him standing down and say opponents are trying to create "chaos" within the party in order to force him to go.
They are stressing the hurdles any attempt to force Mr Brown out faces.
It appears to be aimed at deterring MPs from adding their names to a draft letter due to be circulated on Friday calling on Mr Brown to resign.
About 70 Labour MPs would be required to nominate a specific alternative candidate to trigger a leadership contest.
Pressure has been mounting on Mr Brown after Communities Secretary Hazel Blears resigned on Wednesday, the day after Home Secretary Jacqui Smith said she was leaving the cabinet.
On Wednesday evening Business Secretary Lord Mandelson publicly urged Labour MPs not to sign the letter calling on Mr Brown to resign as prime minister.
Lord Mandelson acknowledged that MPs were in a "grumbly mood" but said forcing out the prime minister would not make things any easier for Labour.
The BBC has seen a proposed letter to Gordon Brown from one Labour MP calling on the prime minister to resign.
It says Mr Brown has made "an enormous contribution" but adds: "We believe that in the current political situation you can best serve the Labour Party and the country by stepping down as party leader and prime minister."
The BBC understands Mr Brown's critics intend to send out some kind of "round robin" letter or e-mail to MPs, possibly on Friday, to test whether there is an appetite for a challenge to his leadership.
On Wednesday Health Secretary Alan Johnson was forced to deny he was ready to take over as party leader.
Asked about Gordon Brown's position Mr Johnson told the BBC: "He is doing the job and there is absolutely no one who could do that job better."
Cabinet ministers Hilary Benn and Andy Burnham have also rallied round the prime minister and attempted to calm speculation he will be forced to quit.
Attempts to gain enough support for someone to stand against Mr Brown for the Labour leadership have failed twice before - when he succeeded Tony Blair in 2007 and last summer.
The talk of a leadership challenge comes amid continued speculation about Mr Brown's planned reshuffle.
The BBC understands Mr Brown met John Reid on Tuesday, but the former home secretary is thought to have turned down the offer of a return to government, although No 10 sources say they talked mainly about football.
And a friend of Chancellor Alistair Darling has told the BBC that he would not accept any other job.
There are signs, too, that David Miliband may refuse to move from the Foreign Office.
Ms Blears had been tipped for the axe in the expected reshuffle. She had been under fire over her expenses claims despite repaying £13,000, following allegations she avoided paying capital gains tax on a property sale.
The Salford MP denied doing anything wrong, as did Ms Smith who resigned after facing revelations about her expenses claims.
In total, four ministers have said they will step down at the reshuffle. The other two are children's minister Beverley Hughes and Cabinet Office minister Tom Watson.