David Cameron: "I would ask the British people to vote no"
David Cameron has refused to give an unequivocal commitment to a referendum on the Lisbon Treaty, after Irish voters delivered a 67% "Yes" vote.
The Tory leader promised a vote on the treaty should his party win the election - but only if it had not been ratified by all EU member states.
He said the Tories "could only have one policy at once", and he did not want to prejudice decisions in other countries.
Gordon Brown said the treaty was "good for the UK and good for Europe".
It aims to strengthen EU decision-making processes by using a majority vote, not unanimity, for more decisions.
It would combine existing roles to make a new post of High Representative for Foreign Affairs and create an EU president (President of European Council).
Ross Hawkins Political correspondent
Now the the treaty has been accepted by the Irish, things have become tricky for the Tories.
If the Tories win the general election next year, and every EU member state has ratified the treaty, then Mr Cameron has said "a new set of circumstances" will apply and he will address those at the time."
So what would addressing those circumstances mean?
For the time being, the Tories refuse to say, and they do not plan to make themselves much clearer at their conference.
The problem for the leadership is that some in the party have no intention of letting the topic rest.
The last thing the Conservatives want at their conference is a public punch-up over Europe - the subject has proved toxic for the party in the past.
Conservative policy remains that, if elected, they would call a referendum on the treaty if it has not been ratified by all 27 member states.
At present the ratification is being held up by Poland - which is expected to sign within days - and a legal challenge in the Czech Republic.
But both countries' parliaments have already approved the treaty.
Mr Cameron told BBC News: "I want us to have a referendum. That is why we are committed to a referendum.
"As long as that treaty is being discussed and debated anywhere in Europe, we will keep fighting for that referendum.
"And if those are the circumstances at the time of the next general election, we will hold that referendum, and I would ask the British people to vote 'No' to that treaty."
When asked why he would not say whether a Conservative government would hold a referendum on the treaty if it was in force, Mr Cameron said he didn't want to risk prejudicing the outcomes on Lisbon in Poland and the Czech Republic.
"You can only have one policy at once," he added.
In a statement made after Irish Prime Minister Brian Cowen announced overwhelming backing for the treaty, Mr Brown said he welcomed the result.
He said: "The treaty is good for the UK and good for Europe. We can now work together to focus on the issues that matter most to Europeans - a sustained economic recovery, security, tackling global poverty, and action on climate change."
But the win for the Irish "Yes" campaign has increased the pressure on Mr Cameron from within his own party to say where he stands on holding a referendum in the UK.
A survey for Conservative Home website of 2,205 Tory party members for the Independent newspaper found 55% would want a referendum on a ratified treaty to "renegotiate EU position", while 29% say they want the treaty declared illegitimate, and a referendum on opting out altogether.
"The findings suggest that the damaging divisions on Europe which destabilised the last Tory government could resurface at the party's annual conference in Manchester," the newspaper suggests.
Liberal Democrat leader Nick Clegg said the Irish vote meant that the Conservatives needed to accept the game was up and they would have to come to terms with the treaty.
The Conservative Party conference starts on Monday.
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