David Cameron is facing a rebellion by Eurosceptic Tory MPs over legislation meant to placate their fears about further transfers of power to Brussels.
The European Union Bill, which comes before the Commons next week, is designed to guarantee a referendum in future before any "significant" loss of sovereignty to the EU. But MPs are concerned that the legislation will make matters worse because decisions on holding referenda will be open to judicial review and so could be overruled by a court.
Labour has already dismissed the Bill as a "dog's dinner", raising the prospect of a cross-party challenge to the Government on January 11.
Bill Cash, the Tory chairman of the European Scrutiny Committee, appealed for support across Parliament for amendments to the Bill which would strengthen the so-called referendum lock.
"The Government has come forward with a proposal which could make things worse," he told The Sunday Telegraph.
"My amendments reaffirm the supremacy of the UK Parliament. I would say this is a matter of national interest, not party politics.
"This is a crucial matter of the national interest and I would hope all MPs would react accordingly and support my amendment."
Philip Davies, the MP for Shipley, said: "The feeling is we have already given away too many powers. If they thought this Bill would buy off the Eurosceptics I think they have made a crass error of judgment."