David Cameron has staked his political career on the outcome of his pledged EU referendum, admitting he saw it as part of his legacy.
The Conservative leader said he wanted to be remembered as the Prime Minister who secured Britain's place in a reformed European Union.
But it emerged one of his party's most senior politicians, London Mayor Boris Johnson, could campaign to take the UK out of Europe. Mr Johnson, tipped to be a future Tory leader, refused to give a guarantee that he would vote to stay in the EU.
At First Minister's Questions, Alex Salmond accused the Conservatives of "heading towards the exit door" on Europe.
It also emerged Mr Cameron did not discuss his poll plan with Scottish Secretary Michael Moore, despite the EU being a key plank of the anti-independence campaign.
Mr Moore insisted Mr Cameron had been speaking as the Conservative leader and that he was "very comfortable" with all of the arguments on the issue, but as a Liberal Democrat would be campaigning for Britain to stay in the EU.
Mr Cameron's stance also came under pressure as the Swedish finance minister Anders Borg described a UK exit as "a big risk" for companies.
At the World Economic Forum in Davos, Switzerland, the Prime Minister was asked if he saw the EU referendum as part of his legacy.
He replied: "I hope I'll be remembered as someone who did everything they could to get the British economy back on track, to strengthen Britain's society and Britain's place in the world, and to secure Britain's place in a reformed European Union.
"I think that is what I want to achieve. There is obviously a huge amount of work in the years ahead, but I feel very confident and positive that, having set out a plan, I think explained to the world, to our European partners, to the British people, British business, everyone can see there is a plan to change Europe for the better, and to secure Britain's place in it."
He added: "And to those who disagree, I would just say, you know, you can't attack a plan if you've got nothing to attack it with. This is the right way forward for Britain. It's in our national interest; it's also in Europe's interest too."
Mr Cameron also said: "We're not putting a list of demands on the table and saying we'll storm off if we don't get them."
His poll pledge had received a cautious welcome in Europe from a number of countries, including Germany. But Finland's Europe minister, Alexander Stubb, warned any renegotiation would be "a long path".
He also appeared to throw cold water on the Prime Minister's plans for renegotiating powers back from Europe, saying: "We have to stick to the bulk of it, but of course we can take a few raisins out of the bun."
Mr Cameron's deputy, Nick Clegg, also cast doubt on the plans. The LibDem leader, whose party opposes the vote, warned "a complete wholesale rewriting of the whole terms of the membership of Britain of the European Union within 18 months flat - I think is wholly implausible".
He said Mr Cameron's aims, as set out in a keynote speech on Wednesday, were "completely vague".
Mr Clegg also poured scorn on the Tory leader's plans to hold the vote in 2017, warning it could take much longer.
He said there was no guarantee that the vote would take place then if his party was in a coalition government with the Conservatives. However, he admitted the Prime Minister's referendum pledge would not be a "deal-breaker" in future coalition negotiations.
Mr Cameron yesterday had talks with prime ministers Mario Monti of Italy, Enda Kenny of Ireland and Mark Rutte of the Netherlands and German Chancellor Angela Merkel in Davos.