Parliament has to have an immediate debate on the EU reform treaty before it can be signed by Prime Minister Gordon Brown next month, MPs have said.
Gordon Brown is due to sign the treaty in two weeks' time
The European scrutiny committee called the wording "ambiguous" and indicated that a "legal obligation" on the UK Parliament could be "inferred".
It repeated its warning that national parliaments could be "marginalised".
Chairman Michael Connarty said the government had to "deliver" a promise to protect UK sovereignty.
The committee's report comes two weeks before the prime minister is due to sign the report next month, to be followed by several weeks of parliamentary debate before it is ratified.
The MPs said they had not been convinced by Foreign Secretary David Miliband's assurances that the final draft of the treaty would contain no obligations on Westminster to the EU.
They expressed alarm that ministers had failed to secure an opt-out from the Charter of Fundamental Rights, saying a UK Protocol would not excuse Britain from the need to comply with rulings by the European Court of Justice.
Mr Connarty, who is a Labour MP, added that Britain's "opt-ins" on justice and home affairs matters would also surrender jurisdiction from the UK courts, while choosing not to opt in would present "new and unquantifiable risks".
The Conservatives say there should be a referendum on the treaty, as was promised with the constitution.
But ministers say there is no need, as the documents are substantially different.
Mr Connarty said: "Despite expressing our deep concerns that the government argue boldly for a form of words that would put the sovereignty of the UK Parliament beyond doubt, there is still ambiguity in the draft treaty on whether a legal obligation is being imposed on Parliament in respect of its proceedings.
"This is not an area in which any ambiguity is tolerable, and we look to the government to deliver on its undertaking."
Another committee member, Conservative MP Bill Cash, told the BBC: "We are saying that we are not satisfied with Mr Miliband's comments and want a debate before anyone signs the treaty."
He added: "This [holding a debate before a treaty] has never been done before. It's on a par with Iraq."
UK Independence Party leader Nigel Farage said: "This is a brave report that deserves better than to be ignored by the prime minister before he scuttles of to Lisbon to ratify this shameful treaty."
Last month, Mr Brown told MPs he had secured "special treatment for the UK in a range of areas" and that Britain would keep opt-outs on foreign policy, labour rights, tax and social security.
"The protections we have negotiated defend the British national interest," he said.
Europe Minister Jim Murphy said: "The reform treaty is a good deal for Britain and helps reform Europe.
"The EU needs to change as it has grown from six countries when the UK joined to 27 today.
"The reform treaty reduces EU bureaucracy and inefficiency as well as speeding up decision making."
The treaty was agreed at an EU summit in Lisbon in October, but all 27 EU member states must ratify it by 2009 for it to come into effect.