How a challenge to Brown's leadership would be mounted

To mount a challenge to Mr Brown, a rival candidate must endure a tortuous process simply to trigger a contest.

First he or she must secure the public support, in writing, of a fifth of Labour’s 353 MPs, then write to Labour’s general secretary announcing the intention to stand.

Rank and file: Labour members must agree to a leadership challenge at the annual conference

Next, a card vote would be taken at Labour’s annual conference and a majority must support a challenge to Mr Brown.

The candidate then has to win a further vote of party members before an election can finally go ahead.

And the complexity does not end there.

The winner is determined by an electoral college voting system which gives equal weight to Labour MPs, grassroots members and affiliated unions.

However, some Labour MPs point to other less formal options for removing Mr Brown.

One is the 'stalking horse'.

A symbolic challenge is mounted by a backbencher, which would not carry enough weight to trigger a formal contest, but might prompt senior MPs and ministers to go public with criticism of Mr Brown.

This in turn might persuade the PM to stand down voluntarily.

Another possibility is the ‘silent coup’. A delegation of senior Cabinet members would advise him that his position had become untenable and offer him the chance to go with grace.

He would then become 'permanently unavailable’, in the words of the party rule book.

Possible replacements would be:

James Purnell, 38: Blairite Work and Pensions Secretary. At ease on television and has impressed in Cabinet, but less popular with some backbenchers.

Ed Balls, 41. Ultra-Brownite Schools Secretary will never stand against his mentor but would seize the opportunity to fill any vacancy.

As Mr Brown’s closest ally, he could be tainted by the PM’s failure.

David Miliband, 42. Foreign Secretary is the choice of Tony Blair. His geeky appearance does, however, worry some.

Jack Straw, 61. Justice Secretary - 62 in August - fancies a shot at the top job before calling it a day. Could be a stop-gap between Mr Brown and a young Turk, such as Miliband or Purnell.

Alan Johnson, 58. Very popular former postman, now Health Secretary.

Could unite warring factions and has the ‘common touch’ to help Labour reconnect with both its heartlands and middle-class voters.

Charles Clarke, 57. One of Labour’s ‘Big Beasts’. His Blairite allegiances mean he is unlikely to win any leadership contest but could be the ‘stalking horse’ candidate.