The right secured by MPs to vote on the chapter before ratification can take effect has posed the prospect of further future embarrassment for the Government if anti-treaty Tories were to combine with Labour and the Liberal Democrats.
However, that threat appeared to recede yesterday as a leading rebel said: 'I suspect we will all vote against the Social Chapter.'
While a senior Government source treated that prediction with caution, the impending legal challenge looked set to become the most pressing issue as it emerged that the case could speed through its High Court, Court of Appeal and House of Lords stages by the end of June or perhaps even earlier - long before the European Communities (Amendment) Bill, the ratification measure, has ground to the end of its parliamentary passage.
The court action stems from the Government's abject and enforced acceptance of a Labour amendment deleting John Major's prized Social Chapter opt-out from the legislation. An opinion by Leolin Price QC, who heads the rebels' legal team, says failure to include the social protocol, which deals with the opt-out, renders the entire process of parliamentary approval of ratification ineffective.
The Government's argument is that it is only 'desirable', not essential, that the protocol is made part of British law.
Mr Price is to decide soon whether to launch a judicial review immediately, or whether to seek a declaration of the true legal position after the Bill receives Royal Assent.
He said yesterday: 'I hope to establish in research still to be done that we don't have to wait.'
In that event, the case could be launched as early as this month and reach a ruling by the Law Lords within weeks. It is likely to cost the rebels between pounds 100,000 and pounds 200,000, depending on the upshot.
Success would herald unthinkable consequences for the Government, and even the possibility that John Major would feel he had little option but to resign.
The latest developments coincide with a warning by Government business managers yesterday that MPs could be prevented from rising for the summer recess until deliberations by peers had ended.
That is calculated to limit resort to disruptive tactics by Lord Tebbit, Baroness Thatcher and their anti- Maastricht cohorts in the House of Lords. James Cran, the MP for Beverley and unofficial whip for the anti- treaty MPs, said yesterday that the call for a referendum would be the centrepiece of the discussions in the Lords. Mr Cran dismissed suggestions that a 'yes' vote in the Danish referendum would amount to a setback for the cause. EC leaders had 'trotted back' after Maastricht expecting the issue to be easily resolved in their parliaments; it had not been. From now on, 'the leaders of the European Community will move as though they are walking on egg- shells', he said.
In the meantime, he confirmed, cash to fund the legal action was in plentiful supply.