The fragile peace of recent months broke down following reports, which he has refused to deny, that David Heathcoat-Amory, a Treasury minister, will stand down in the ministerial reshuffle due this week, to campaign against the single currency.
Yesterday a number of other sceptics broke cover, demanding that John Major rule out membership of a single currency in the next Parliament. Significantly, they included James Cran, who holds a government post as Parliamentary Private Secretary at the Northern Ireland Office.
Steve Norris, a junior minister at the Department of Transport, is also expected to back the demand when he steps down from office in the reshuffle. And John Redwood, last year's leadership challenger, is already pressing the same case.
This leaves Mr Major's delicate balancing act on Europe - which involves postponing a decision until nearer the relevant time - looking wobbly. Mr Cran declared: "The Government is going to find it irresistible to rule out a single currency in the lifetime of the next Parliament."
Michael Brown, a former whip, said: "If the Prime Minister is re-elected, he will not be able to get a single currency through. I will not vote for it, whatever the he says."
Euro-sceptics now argue that the Chancellor, Kenneth Clarke, is increasingly isolated. One said: "The Chancellor is holding the party to ransom. There is a tremendous opportunity for Mr Major to say to Ken Clarke: 'If Heathcoat-Amory has to go, you should go too'."
Meanwhile Lord Howe, the former Chancellor and Foreign Secretary, hit back at the Euro-sceptics, accusing them of provoking a "huge factional split".
Conservative Central Office is bracing itself for restiveness from activists at the party conference in October. At Westminster many backbenchers are planning statements against EMU in their election addresses.
Mr Redwood claimed that the Government had already tacitly accepted that it could not meet the timetable for introducing the Euro. "To qualify for a single currency, the UK will have to legislate to make the Bank of England more independent," he said. "The fact that the Treasury is not doing so implies that they don't wish to be ready for 1998, when the decision has to be taken. It would be better to rule out joining in the next parliament, and wait and see how things develop."Reuse content