Mr Major must clear two hurdles to win in the first ballot on 4 July. He needs the votes of a majority of the 329 Tory MPs, at least 165 votes. In addition, he must win by a margin of at least 15 per cent of the parliamentary party, which means he must be 50 votes ahead of the nearest challenger.
Nominations close at 12 noon next Thursday. If a second ballot is required, it will take place on 11 July, a week after the first ballot. In this vote, there is no 15 per cent rule - a simple majority of Tory MPs is all that is required to win. If no candidate wins 164 votes, the top two candidates go through to a third ballot two days later.
In the event of a tie, "and unless the two candidates are able to inform the chairman of the 1922 Committee that they have resolved the matter between themselves", a fourth ballot will be held on 18 July. New candidates are allowed to enter the contest before the second ballot, and candidates may withdraw at any time.
Conservative leadership elections are organised by the 1922 Committee, which consists of all Tory MPs who are not in the Government.
The rules were changed after the 1990 contest to introduce the requirement that a challenge to an incumbent leader could only be triggered by 10 per cent of Tory MPs publicly requesting an election.
But Mr Major's decision to resign from the party leadership to force a contest, rather than ask the 1922 Committee to change the rules again to bring forward November's annual election, means that he is not protected by the 10 per cent threshold.
Under the rules, there could be another leadership election this November. But Ian Lang, Secretary of State for Scotland and a member of Mr Major's campaign team, said last night that the executive of the 1922 Committee had "made it clear" that there would be no contest in November. This means yet another rewrite of the rules.