In 1990 an obvious alternative prime minister was outside the Cabinet and ready to strike: Michael Heseltine. As one Conservative MP put it: 'In 1990 there was always a way out even if it was a difficult one. We could retreat from Stalingrad. It was not too late. This time everyone feels despair because they cannot see a way out.'
The significance of the events of last week was that, although no escape route has become clear, many Conservative MPs have actively begun looking for one.
They do so in the knowledge that Mr Major faces a series of big tests unlikely to improve his position: the local elections on 6 May, the Eastleigh by-election, the European elections on 9 June and the findings of Lord Justice Scott's inquiry into the arms-to- Iraq affair.
So how might John Major eventually be removed from office? And how plausible are the various possibilities? The main alternatives include:
MEN IN GREY SUITS
After electoral disaster and a damning Scott inquiry, Lord Whitelaw; Lord Wakeham, the leader of the Lords; Sir Marcus Fox, chairman of the 1922 Committee of backbench MPs; Douglas Hurd, the Foreign Secretary; Lord Mackay, the Lord Chancellor, and Richard Ryder, the Chief Whip, (or a combination of them) call on Number 10 and tell Mr Major it is time to step down.
Comment: The 'men in grey suits' form a powerful and popular image, but their existence is largely mythical. Grandees have, in the past, helped to ease Conservative leaders out of office but usually after a general election defeat. In 1974, for example, it was partly pressure from the 1922 Committee, chaired by Edward du Cann, which ensured that Edward Heath had to face re-election. He lost.
Mr Major is in mid-term and the 'magic circle' which once decided issues of leadership in the Conservative Party is well and truly dead. Few of today's grandees look up to delivering the message. Lord Whitelaw is a powerful figure in the Lords but is a political generation older than most of the Cabinet. Lords Wakeham and MacKay play important roles in Cabinet policy disputes but not in party matters. Neither Mr Hurd nor Mr Ryder, both loyalists, are likely to want to cast the first stone against Mr Major (despite one MP's observation that 'Douglas always seems to wear a grey suit'). Backbench sentiment would have to be overwhelming for Sir Marcus to deliver such a message - and the only way to gauge that is by a leadership contest.
Verdict: Just possible (particularly if combined with 'sudden illness') but potential assassins lack the killer instinct.
After humiliating losses in and around London in the local elections, at Eastleigh and in the European elections, Mr Major succumbs to an illness, stepping down voluntarily. He takes the same exit as Anthony Eden after Suez or Arthur Balfour in 1911.
Comment: Sitting prime ministers rarely leave without being forced to do so. In any case, so low are the expectations for the forthcoming elections, that the Tories may perform slightly better than predicted (particularly in May). Mr Major's health and stamina have survived the test of time. As one Tory MP put it: 'I doubt if he is capable of riding off into the sunset. He is 51 years old and has no other interests apart from watching cricket. His instinct would be that it had all been got up by the press'.
Verdict: Just possible but it would be out of character for Major.
After more politicial humiliations, senior Cabinet ministers threaten to resign unless Mr Major goes.
Comment: After Mr Heseltine's first ballot challenge in 1990, Margaret Thatcher could have gone on to the second ballot. The desertion of the Cabinet decided her not to do so. But the situation is different now. The right of the Cabinet, whose candidates are weaker than those of the left, would be in a quandary. The left might be split over its candidate. And Mr Major does not bully colleagues or interfere in departments, as did Lady Thatcher or Sir Anthony Eden. Some right-wing ministers, such as Peter Lilley and John Redwood, might prefer a weak Major to a strong Heseltine or Clarke. Some others would calculate they would almost certainly be sacked by a new incumbent.
Verdict: Highly unlikely; most ministers prefer the devil they know.
A COMMONS VOTE
Large numbers of Tory MPs vote against the Government on an important motion or piece of legislation.
Comment: The precedent most often quoted is that of Neville Chamberlain who, in what amounted to a vote of confidence, saw his Commons majority reduced from 200 to 80 in 1940. But Mr Major's majority is too small to allow a significant rebellion without precipitating a General Election - which, given the party's present standing in the polls, Tory MPs would be desperate to avoid.
It is possible that Mr Major could be forced to rely on large numbers of Opposition votes or abstensions to get a piece of Europe-related legislation through the Commons. Two such bills are in the offing: one to increase Britain's contributions to the EU and one to approve the arrangements for enlargment. . But Mr Major could threaten to turn the issue into a vote of confidence as he did over the Maastricht Bill, once more leaving his opponents staring at the unwelcome prospect of a General Election. In any case, the two bills now look as if they will be deferred until the next session of Parliament - leaving Mr Major safe at least until November.
Verdict: Highly unlikely; too risky for most backbenchers.
Eurosceptic ministers decide that they can no longer stomach government policy towards Brussels and walk out.
Comment: Cabinet right-wingers stomached Maastricht; they also accepted last week's decision to back down on the demand that the blocking vote in the EU Council of Ministers should remain at 23. They seem unlikely to act now. Several colleagues lower down the ranks, including Michael Forsyth and Eric Forth, have less to lose. However, their bluff has already been called over Maastricht and they may fear a Heseltine or Clarke premiership more than they despise the current administration.
Verdict: Unlikely; Eurosceptics have nothing to gain.
A LEADERSHIP CHALLENGE
In November, a backbench MP challenges Mr Major in a leadership contest, receiving enough votes in the first ballot to force him to retire.
Comment: This was how Margaret Thatcher was overthrown in 1990, when Mr Heseltine challenged her. But, this time, Mr Heseltine is inside the Cabinet - and, therefore, according to the unwritten rules, unable to challenge - and there is no figure of comparable stature on the backbenches.
So Tory MPs wishing to be rid of the Prime Minister must find a 'stalking horse' - a candidate who is not expected to win, but who can flush out enough votes to show the extent of dissatisfaction with Mr Major. This was the role that Sir Anthony Meyer performed in 1989. In that case, it was another year before a serious leadership challenge was mounted and the same may prove true in this case. But if as many as 100 MPs voted for the stalking horse or abstained, Mr Major would probably stand down immediately, thus starting a new contest.
Any MP needs the backing of 10 per cent of the party (34 MPs) to force a contest. The most obvious candidates are Tony Marlow, who last week called on Mr Major to go; Teresa Gorman; Kenneth Baker or Norman Lamont - although the ex-Chancellor's need for a seat following boundary changes may limit his scope for trouble-making.
Some MPs believe that any of the possible candidates would be lucky to get more than about 60 votes and/or abstentions because all have their strong enemies and that although around 100 MPs would vote against Mr Major that figure could only be mobilised if credible alternatives came into the race. Others argue that many Conservative MPs would seize the chance of abstaining no matter who was the candidate.
Verdict: The most likely choice for getting rid of this PM.