TAOISEACH Brian Cowen must have heaved a major sigh of relief when the result was announced in Dublin Castle last night. But across the water prime minister-in-waiting, David Cameron, may not have been quite so pleased.

If Ireland had voted 'No' on Friday, Lisbon would have been dead and the Conservative Party leader would have been off the hook in relation to his promise to hold a referendum on the treaty when he entered 10 Downing Street.

But with Ireland voting 'Yes' and Poland, Germany and the Czechs likely to formally ratify Lisbon well before the British general election next year, Cameron is left in something of a quandary.

Up to now, Cameron has said that "if the treaty is still alive, if it is still being discussed and debated anywhere in the EU, then we will give you that referendum, we will name the date during the election campaign. We'll hold the referendum straight away and I will lead the campaign for a No".

But he has not promised to hold a referendum if the treaty has already been ratified across the EU, saying that in this scenario a "new set of circumstances [apply] and I will address those at the time". While moderates in the Conservative party believe that in these circumstances, it's time to accept reality and live with Lisbon, others will be pushing Cameron to hold a referendum even though the treaty has been ratified.

But effectively tearing up a ratified treaty would be an extraordinarily risky course of action for a new prime minister. While it would appeal to euro-sceptics in his own party – who backed Cameron during the Tory leadership contest of 2005 after he gave them guarantees on Europe – and beyond, it would lead to an almighty row with Britain's EU partners and would inevitably be portrayed as a de facto plebiscite on Britain's entire EU membership.

BBC's respected Europe Editor Gavin Hewitt summed it up well on his blog last week: "Here's the question: Would a new prime minister taking office with a vast budget deficit, and with a commitment to mend 'broken Britain' spend precious political capital and energy on a ferocious row with the likes of [German chancellor Angela] Merkel and [French president Nicolas] Sarkozy?

Cameron didn't get to the point where he looks likely to become the first Tory leader in 17 years to win a general election by expending unnecessary political and energy, so the logical answer to Hewitt's question is no.

The Tory leader has clearly signalled he would not consider a move that could lead to Britain leaving the EU. "I want us to be in the European Union. We are a trading nation, we should be co-operating with our allies and friends in Europe over things like the environ­ment and crime, of course we should."

Although Cameron's aides insist his policy has not changed, some believe he is preparing the ground to move away from a referendum ? a course of action which could alienate the Eurosceptic wing of the party and re-open old divisions within the Conservatives over Europe. There have already been predictions that Cameron will come under pressure from both sides of the argument at this week's Conservative party conference.

All of this would have been avoided if only the Irish had voted 'No' on Friday. Now that we haven't, to rejig an old historical aphorism, Cowen's opportunity looks set to prove Cameron's difficulty.