The Conservative chairman, Sir Norman Fowler, told the Young Conservative Conference in Southend - the constituency represented by the arch-Tory Eurosceptic Sir Teddy Taylor - that 'no one who puts Britain's interests first could ever vote for the Social Chapter'.
His comments came as rebels warned privately that, in order to wreck the treaty, they would be willing to vote for a Labour amendment inserting the Social Chapter - on which John Major negotiated an opt-out for Britain - into the Bill now going through Parliament.
The burdens of the Social Chapter would increase unemployment, said Sir Norman. 'The importance of our stand against the Social Chapter was shown again last week when the Hoover company closed a factory in France and moved it to Scotland.'
However the speech, produced a defiant reaction from the rebels. James Cran, Conservative MP for Beverley, said: 'We did not start this argument just to chicken out at the last minute. The chairman of the Tory party can make his speech but, at the end of the day, we will have to take our decision about how we kill the treaty.'
The Eurosceptics will try to reach a collective agreement on whether to defy the Government by voting for Labour's amendment 27 - the one on the Social Chapter. But they have not yet met to discuss the issue.
The Tory whips know that backing the Social Chapter, which spells out minimum working conditions, would be difficult for right-wing Tories to justify to constituency associations.
However, given the Government's low majority, it is still viewed by ministers as the area most vulnerable to an amendment. Labour and the Liberal Democrats say their MPs will give solid backing to the amendments, which they argue would not wreck the Bill.
Sir Norman said that the Liberal Democrats, whose votes could help to scupper the Government's majority, were leftwing. 'They throw in their lot with Labour whenever they get the chance,' he said, apparently overlooking their support for the Government in last year's debate.
The Labour leader, John Smith, he added, had made 'torpor an act of policy'.