Tony Blair has hinted he could hold a second referendum on the European constitution if the public said "no" the first time round.
Blair says Britain would be left behind under Tory plans
Tory leader Michael Howard seized on the words, which came in his clash with Mr Blair in a rowdy House of Commons.
Mr Blair said he would enter new talks if voters rejected a first treaty deal.
In fierce exchanges, Mr Howard mocked ministers' U-turn on holding a vote. But the PM rejected his call for a televised debate on the constitution.
Later, Mr Howard ruled out ever signing up to a European constitution if the referendum produced a "no"
Does no mean no?
At prime minister's questions, MPs from both sides heckled and jeered as the two leaders slugged out their opposing positions on the EU constitution in the opening salvoes in a campaign expected to last for months.
The Tory leader argued a televised debate would help the parties "explore all the issues".
The two main groups for either side of the campaign can spend up to £5m
Each will be eligible for £600,000 from public funds
Other groups or individuals are barred from spending more than £500,000
Campaign period is normally 10 weeks
But Mr Blair retorted that once the constitution was approved by Parliament, "we'll have the debate in the country".
He faced a series of questions about what he would do if there was a "no" vote while he was still in office.
Mr Blair said: "We will be in exactly the same position as Ireland after its rejection the first time around of the Nice Treaty.
"Which means that if we were in government, we would sit down and have to discuss the way forward with other European countries."
Ireland rejected the Nice Treaty in a referendum only to be ratified by a repeat of the poll 16 months later.
Later, Mr Blair's official spokesman told reporters: "If there was a 'no' vote the government would obviously still want to be part of Europe and want to be a positive part of Europe.
"But you should note there is an obvious logic to what the prime minister said to the House today.
"What nobody should underestimate is the difficulty this country would be in if the rest of Europe had made a decision."
COUNTRIES HOLDING OR LIKELY TO HOLD A REFERENDUM
The comments prompted a fresh offensive from the Tories.
In a speech to the British Chambers of Commerce, Mr Howard said the government planned to renegotiate the treaty if there was a "no" vote.
"In other words, if the British people did not vote the way he wanted, Tony Blair would make them vote again until they did," he said.
STEPS TO A REFERENDUM
Constitution expected to be agreed in June
Parliamentary approval may be sought before a vote
It could also take place alongside referendums for English regional assemblies this autumn
A vote could also be held at the time of the general election predicted next spring
It is also possible a vote will take place after the next general election
As the battle lines were set out, the prime minister suggested the Tory stance would transform Britain's relationship with the EU and leave other nations going ahead without it.
In a letter to Mr Howard, he accused the Tory leader of dodging the question of whether he would reject a treaty in its entirety or renegotiate if there was a "no" vote.
Responding in what is already becoming a war of words, Mr Howard said: "If the British people were to vote 'yes', a Conservative government would accept the constitution.
"If the British people were to vote 'no', a Conservative government would veto the constitution: and we would not agree to any new treaty which establishes a constitution for the European Union."
Roger Knapman, leader of the UK Independence party, said he was convinced the "no" campaign would win the battle, which would mean "we're out of the EU".
Liberal Democrat leader Charles Kennedy said the vote announcement gave an opportunity at last to "settle an issue that has bedevilled two generations of politicians".