Challenge triggered by just 25 signatures

A Tory leadership contest would be triggered by a vote of no-confidence in Iain Duncan Smith, with 15 per cent of the party's MPs signing a letter requesting a motion.

On the basis of the party's parliamentary representation, 25 MPs would need to sign the letter to Sir Michael Spicer, chairman of the 1922 Committee. For MPs in marginal seats, the question of whether they will survive the next election under Iain Duncan Smith may be a key factor in prompting them to sign the letter.

The 1922 Committee would then, in a secret ballot, decide the motion of confidence in Mr Duncan Smith. For Mr Duncan Smith to survive he would have to get 50 per cent of MPs' votes, plus the vote of one more, amounting to 83 votes or a simple majority of the parliamentary party. If he failed to achieve this, his career as leader would be over and a leadership contest would be triggered.

In the next stage, potential candidates for leader of the party would line up. Any MP except for Mr Duncan Smith could stand and their parliamentary colleagues would vote in a series of rounds to eliminate the least popular candidates.

The two MPs achieving the most votes would have to face the Tory rank and file. After an exhausting tour of Tory constituencies they would be subject to a postal ballot involving all paid-up members of the Conservative Party. This vote establishes which of the two contenders will become the leader.

The rules for a leadership challenge were established by William Hague, and dispensed with the need for a "stalking horse" candidate who stood little chance of winning but acted as a barometer of the leader's popularity among his colleagues.