The Prime Minister, Gordon Brown, faced a sharp rebuff over Europe after a Labour-dominated committee attacked the "secrecy" of European reform.
The Commons European Scrutiny Committee also contradicted the repeated statements of ministers that the proposed EU reform treaty significantly differed from the planned EU constitution, which was abandoned after it was rejected by French and Dutch voters.
A report by the committee said: "Taken as a whole, the reform treaty produces a general framework which is substantially equivalent to the constitutional treaty." It also warned that Britain may not be able to maintain its "red lines" over important policy areas.
The report, published just days before European leaders meet in Lisbon to thrash out a final deal on the reforms, gives ammunition to sceptics who say the treaty should go to a referendum.
Mr Brown said: "If our red lines are not achieved, I have always said we will either veto it [the EU Reform Treaty] or say there has got to be a referendum. I believe that the red lines will be achieved and we will show that we have managed in the course of our negotiations to persuade our European partners that what we want is not only right for us but right for them."
The committee's Labour chairman, Michael Connarty, criticised the Portuguese European presidency for producing a text of the agreement just 48 hours before the European Council meeting in June.
Mark Francois, the Tories' Europe spokesman, said: "It is now crystal clear that the two documents are essentially the same and therefore Gordon Brown is morally bound to offer the people of the country the referendum he promised them."Reuse content