A UK referendum on the EU constitution moved a step closer after MPs backed a bill paving the way for the vote.
UK voters will decide on the EU constitution
The Tories tried to block the proposed law saying there should have been two bills, one for the referendum and one adopting the treaty into UK law.
The constitution was signed last year to change the way the European Union works now it has 25 members.
Foreign Secretary Jack Straw said that rejecting the constitution would prompt a crisis.
Spring 2006 vote?
Shadow foreign secretary Michael Ancram said the treaty was a step closer to a European superstate.
The Lib Dems are in favour of the constitution and of voters having the right to decide the issue.
The constitution, designed to streamline EU decision making, was signed last October but has to be ratified by all EU states, with some holding referendums.
Ministers suggest the UK vote will not take place until spring 2006, although the Tories want it held earlier.
The European Union Bill now goes to committee stage in the House of Commons after it was passed by a majority of 215 MPs.
The referendum question will be: "Should the United Kingdom approve the treaty establishing a constitution for the European Union?"
Mr Straw said the constitution signalled "thus far and no further" by clearly setting out the limits of EU powers.
He said the constitution would make the EU better managed and more accountable to national parliaments.
The UK would keep its veto on tax, social security, foreign policy, defence and its budget rebate, he said.
But replacing the veto with qualified majority voting in other areas could produce reforms the UK wanted.
Mr Straw argued: "This is a constitution which works in Britain's interests. If we were then to say 'no', it would leave Britain literally isolated, it would leave Britain weaker."
He admitted opinion polls suggested most people opposed the constitution but pointed to evidence of many people being undecided.
Later, he told MPs that Tory calls for renegotiating EU treaties were a "fantasy".
Mr Ancram insisted determined negotiation could win changes and other countries, such as the Netherlands, wanted powers returned to national parliaments.
"This constitution is the irreversible first step towards a European state - it's the gateway to a country called Europe which I don't believe the people of this country want to go to," he said.
He questioned why a new European foreign minister was needed if the UK was really to keep control over foreign policy.
Lib Dem foreign affairs spokesman Sir Menzies Campbell said the constitution was "long overdue and defines the powers and responsibilities of the EU and of national parliaments".
"It is in Britain's interests that it should be ratified."
He criticised the government for missing too many opportunities to make the pro-European case.
The EU's benefits in terms of securing peace, jobs and travel should not be allowed to "go by default", he argued.