The government is facing new pressure to hold a referendum on the proposed EU reform treaty after a Labour-led committee of MPs found it was "substantially equivalent" to the EU constitution thrown out by the Dutch and French.
The Commons European scrutiny committee said ministerial claims that the new treaty no longer had the characteristics of a constitution were "less than helpful".
In a report today it said these claims are "likely to be misleading", as they might suggest that the treaty is less significant than the constitution, and it was down to the government to supply evidence to support its assertion.
Comparing the two documents, the report found that all the innovations introduced by the constitution were contained in the new treaty, with the exception of EU symbols like a flag or anthem.
The government had promised to hold a referendum on the constitution, but this became unnecessary when it was ditched after France and the Netherlands voted to reject the document.
The government has repeatedly insisted there is no need to hold a referendum on the new treaty - which is due to be signed by EU heads of government in the Portuguese capital Lisbon at the conclusion of an intergovernmental committee (IGC) this month - because it is "substantially" different from the previous document.
The report by the committee, nine of whose 16 members are Labour MPs, has bolstered Conservative claims that the government is "morally bound" to hold a referendum on the treaty.
The committee also warned that the special UK opt-outs and protocols secured by the government to protect its so-called "red lines" may not prove effective in practice.
And it criticised the "secretive" process under which the draft was compiled.
In another awkward conclusion for the government the committee also found that a requirement in the treaty for national parliaments to contribute to "the good functioning of the union" might contradict Britain's 1688 Bill of Rights, which protects parliament from being put under legal obligations by any outside body.
It raised "a serious difficulty of a constitutional order", the committee said.
"In our view, the imposition of such a legal duty on the parliament of this country is objectionable as a matter of principle and must be resisted."
The report added: "What matters is whether the new treaty produces an effect which is substantially equivalent to the constitutional treaty.
"We consider that, for those countries which have not requested derogations or opt-outs from the full range of agreements in the treaty, it does."
The report warned that - even with the measures the UK had secured at a summit in June to preserve its "red lines" on foreign policy, labour legislation, common law and tax and social security systems - the UK might find itself effectively signed up to the provisions set out in the old constitution.
"Taken as a whole, the reform treaty produces a general framework which is substantially equivalent to the constitutional treaty," it said.
New provisions in the treaty to allow national parliaments to object to measures proposed by the unelected European Commission "add very little by way of democratic control over the commission and the EU institutions", said the committee.
The MPs were scathing about the way in which the reform treaty was drawn up, with draft texts available to member governments only 48 hours before the June summit and national parliaments "marginalised" from the process.
The committee's Labour chairman, Michael Connarty, said: "The European Council claims it wants to provide EU citizens with 'full and comprehensive information' during the IGC.
"However, the essentially secret drafting process conducted by the presidency, combined with texts produced at the last moment before pressing for agreement, could not have been better designed to marginalise the role of national parliaments."
The Conservative spokesman on Europe, Mark Francois, said: "Gordon Brown told the BBC's Andrew Marr at the weekend that if we were still dealing with the 'old proposal' he would offer a referendum.
"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."
The chairman of the I Want a Referendum campaign, Derek Scott, a former adviser to Tony Blair, said: "This report blows a huge hole in the case against a referendum."
But the minister for Europe, Jim Murphy, repeated the government's rejection of the need to put the document to a vote.
"I have explained previously before the committee, the government notes that the reform treaty is significantly different to the old constitutional treaty in intent, form and substance. All 27 EU leaders have agreed that 'the constitutional concept has been abandoned."
An alternative report tabled by the Conservative committee member Bill Cash, stating that the treaty "requires a referendum of the electorate of the United Kingdom" was defeated by 7-3, with three Tories on the committee overruled by Labour and SNP members.
Mr Cash said: "The government has deceived the British people. Tony Blair has left Gordon Brown a poisoned chalice. The reform treaty is substantially the same as the constitutional treaty.
"The government has broken its promises and must now reject the treaty, and if it will not, must hold a referendum for the tens of millions denied one on Europe since Harold Wilson in 1975 and must stop the charter by passing Westminster legislation overriding the European Communities Act 1972."