In the most dramatic gamble of his 36-year political career, the Prime Minister seized the initiative in the internal warfare crippling his party by provoking what could be the first Tory leadership contest since he replaced Margaret Thatcher in 1990.
As Mr Major announced his decision in a press statement in the garden of 10 Downing Street, some of his closest Cabinet allies were already putting in motion contingency plans for a leadership campaign under the management of Lord Cranborne, Leader of the Lords.
The period for nominations opened last night and will end on Thursday. With the full Cabinet rallying publicly behind him last night and the entire 18-strong executive of the 1922 Committee agreeing to sign his nomination papers, it is now up to one of a series of potential right wing "stalking horses" to seek a proposer and seconder to back a challenge.
Michael Heseltine, widely canvassed as his most powerful leadership rival, said within minutes of Mr Major's announcement: "I have made position very clear. I shall support him."
Douglas Hurd, the Foreign Secretary - and the subject of fresh speculation that he would step down in the reshuffle which Mr Major intends to implement if he survives - said it was a "brave step by a brave man. He is the best Prime Minister for our country and I wish him all success." Mr Hurd said he would be "enthusiastically" participating in Major's campaign if one became necessary.
Yesterday's astonishing turn of events began with a letter in the Times from Sir Patrick Mayhew which warned that the constant "commotion" against Mr Major's leadership was putting the Northern Ireland peace process at risk.
Mr Major's move had been immediately precipitated by Baroness Thatcher's criticisms of his administration in interviews to promote the second volume of her autobiography; the mauling which Mr Major suffered when he met the Euro-sceptic Fresh Start group 10 days ago; and the leadership crisis which Mr Major faced while at the G7 summit.
The 1922 executive also agreed last night to re-examine the party rules - drawn up in 1965 and amended after Lady Thatcher's fall - in a move which could rule out any subsequent challenge between now and the general election.
But while Mr Major's pre-emptive strike - described last night as "brave", "bold" and "courageous" by a wide spectrum of Tory MPs - could indeed lay the endless leadership speculation to rest, it also carries the inevitable risk that a "stalking horse" could attract a large enough number of votes and abstentions to further undermine his authority or, at the very worst, precipitate a second ballot. Mr Major himself told close aides yesterday that he recognised the "high risk" involved but that it was impossible to go through a summer and party conference season with his premiership being constantly undermined.
Against that eventuality, Lord Cranborne is heading a campaign team which includes Brian Mawhinney, the Transport Secretary; Ian Lang, the Scottish Secretary; the Home Secretary, Michael Howard; and Tony Newton, Leader of the Commons. It headquarters will be the house of the former MP Sir Neil Thorne in Westminster.
While Mr Major's close allies have been suggesting that a "stalking horse" would have to secure the 164 votes needed to unseat him, other senior party figures suggested last night that a total of more than 100 votes and abstentions against Mr Major would precipitate a fullblown leadership crisis.
Norman Lamont, the former Chancellor; Tony Marlow, the maverick right- winger; Edward Leigh, former industry minister; and John Carlisle, have all been named in recent weeks as possible stalking horses. Mr Carlisle, one of the least credible potential contenders, refused to rule himself out. Mr Lamont remained silent on the leadership but launched a blistering attack on Mr Major's European policy in today's Times.
Mr Marlow said last night that Mr Major was a "loser" and added: "If he isn't challenged it will be a disaster." He claimed a decision would be taken next week about whether he should stand.
But the audacity of Mr Major's move is underlined by the tactic he has adopted. By stepping down, he has ensured that a challenger needs only a proposer and a seconder. If he had waited until November, it would have required 33 names of MPs to precipitate a leadership contest.
Lady Thatcher predicted Mr Major's survival and added: "I think it is a good thing that he did this. It shows that he cares."
Tuesday 4 July
Only if "stalking horse" candidate emerges
Tuesday 11 July
Required if Major fails to receive at least 164 votes, and 50 more than his nearest rival.
Thursday 13 July
Held if no candidate has a majority.
Tuesday 18 July
Held in event of a tie