UK Prime Minister Gordon Brown has belatedly signed the EU reform treaty, having missed a ceremony attended by leaders of the 26 other member states.
Critics claim the treaty means Britain surrenders power to Brussels
Mr Brown delayed his trip to Lisbon so he could appear before a Commons select committee scrutinising his government.
He promised the committee there would be a full debate in Parliament on the 250-page text but no referendum.
The Conservatives said Mr Brown's "diary clash" did not reflect well on him, making him appear "gutless".
"If he believes this treaty is the right thing for the country then he ought to have the guts to go to the actual signing ceremony," Shadow Foreign Secretary William Hague told BBC News 24.
"We had this chronic indecision in Downing Street about what the prime minister would do. I don't think that's a very good advertisement for prime ministerial decision-making."
Kenneth Clarke, the pro-European former Conservative chancellor, said Mr Brown's "stunt" proved he was "not very good at international diplomacy" and was more concerned with newspaper headlines.
It was "a foolish way of going about defending a treaty which he's taken part in the negotiations of", Mr Clarke told BBC News 24. "He's behaving rather childishly."
The Liberal Democrats said Mr Brown's absence raised "serious questions".
Lib Dem leadership contender and ex-MEP Chris Huhne criticised "inept and peevish behaviour that leaves Gordon Brown's reputation for honest dealing with our EU partners hanging by a thread".
UK Independence Party leader Nigel Farage said he spoke to Foreign Secretary David Miliband in Lisbon just moments before he signed the treaty and repeated his demand for a referendum, receiving only "a hollow laugh" in reply.
Mr Farage said: "This is just about the most thoroughly dishonest political process I have ever been witness to.
"This is a constitutional treaty with profound, far-reaching implications and for the British Government to pretend it is something it isn't and deny us a referendum is monstrous."
Mr Miliband stood in for Mr Brown for the signing in the Portuguese capital.
Mr Brown has said there was no need for a referendum as the treaty was different from the constitution rejected by voters in France and the Netherlands in 2005.
But he told the Commons liaison committee there would be "more scope for Parliament to debate some of these issues than there has been in the past".
The treaty has now been signed by all 27 members of the EU
The UK will give up its veto in many policy areas as the EU introduces more qualified majority voting, but Mr Brown said the changes were in the country's interests.
"Some of them are minor and procedural and the other ones are in Britain's interest and if they are not we have usually got an opt-in or an opt-out to decide whether we wish to be part of it," Mr Brown told the committee.
He said MPs would have the final say over whether Britain opts out of justice and home affairs legislation, when it is taken over by the EU.
But MPs would not be able to vote on which parts of the treaty Britain wanted to opt in to as decisions on that had to be made within a "three-month window", he added.
"It will have to be a matter for the government to make that decision on the basis of what we know to be the best interest of the country but the general debate we will have in the House of Commons," Mr Brown said.
He said he was sure some "ingenuous" MPs would find ways to introduce amendments to the bill ratifying it, but he declined to go into detail about the wording of the bill, saying it would be published "very soon".
The treaty will greatly alter the way members govern themselves. It creates an EU president and a vastly more powerful foreign policy chief for the union's 27 nations.