It would risk derailing the Conservative campaign by directing media attention to the Tories' most divisive weak spot - Europe - and such an open breach of collective ministerial responsibility would prompt demands for ministerial rebels to be sacked.
A senior government source said last night that rifts over Europe were more damaging the closer they broke to an election. In an election campaign itself, they would be "seriously troublesome".
He said that while aberrant behaviour could be expected from some "green" candidates, disciplinary action would have to be taken against ministers who issued electoral addresses that flew in the face of the national party manifesto and damaged party credibility.
Unlike MPs, who automatically lose their jobs once Parlia- ment is dissolved for an election, ministers stay in office until a new government takes over - and could risk instant dismissal for a breach of collective ministerial responsibility. Labour would demand nothing less.
It has been estimated that as many as two-thirds of Tory candidates could locally dissociate themselves from the national manifesto line; that no decision can be made on sterling entry into the single currency until firm terms have been agreed, after the election.
Tory Euro-sceptics want the Prime Minister to stand on a platform defending the pound, and despair that this week he deepened his commitment to the wait-and-see line.
But senior Conservatives believe the tensions of the election campaign will snap the rigid, wait-and-see agreement that was forced on Mr Major by Michael Heseltine and Kenneth Clarke on Tuesday.
A parallel is being drawn between the current Tory truce and the 1983 Labour deal on nuclear disarmament which so effectively destroyed the credibility of Michael Foot's first and last election campaign as Labour leader.
In that campaign, deputy leader Denis Healey and former prime minister James Callag-han undermined the manifesto commitment to a "non-nuclear defence policy" every time they reiterated their views.
The Conservative Euro-sceptics believe that a number of cabinet ministers will be tempted, during the election campaign, to cast doubt on British participation in the first wave of the single currency. By delivering coded challenges to the wait-and-see policy enshrined in the Tory manifesto, they would attract headlines while avoiding disciplinary action.
The names of Michael Howard, the Home Secretary, and Michael Portillo, Secretary of State for Defence, are being nominated as the most likely ministers to step over that line.
Every time that happens, the party will look less and less credible. The Westminster suspicion is that once Mr Major loses that essential credibility, and risks losing to Labour, the gloves will come off for the Tory leadership contest that is certain to follow election defeat.
Euro-sceptics will also grasp a provocative new draft of the Maastricht treaty, seen by The Independent, which calls for the abolition of national border controls.
The document is grist to their mill, even though Britain could ultimately opt out if a majority of countries agree to the text, which has been produced by the Irish ahead of next week's Dublin summit of European leaders.
Tory pro-Europeans are determined to resist further attempts to reopen the single currency policy. The Positive European Group of Conservative MPs has brought forward its weekly meeting from Monday of next week, to tonight, and according to senior officials it will be discussing how to consolidate the Prime Minister's acceptance that existing policy will not be changed this side of an election.
One source said: "The first step, really, is to make sure there is no further back-sliding."
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