David Cameron is to set out his vision for Britain's future relations with the European Union in a long-awaited speech in the Netherlands on Friday.
The Prime Minister is expected to announce plans for a referendum on a new settlement with Brussels after the 2015 general election.
He has rejected calls for an immediate in/out referendum on British membership, which he said would present voters with a "false choice". But he said it would be right to seek the "fresh consent" of the British people after negotiating a new settlement for the UK. Speaking to BBC Radio 4's Today programme, Mr Cameron stopped short of explicitly promising a referendum, but said consent would be sought in a "straightforward way".
Mr Cameron has been caught in an increasingly ill-tempered row within his party, with some Tory backbenchers calling for a public vote on the EU, while business leaders and grandees including Lord Heseltine have warned of the dangers of UK withdrawal. Cabinet minister Kenneth Clarke is expected to share a pro-EU platform with Labour's Lord Mandelson later this month, and Mr Cameron is coming under pressure from Liberal Democrat coalition partners not to lead the UK towards the exit door.
Labour said the PM had lost control of the agenda on Europe, and appeared to be putting the demands of party loyalty ahead of the national interest.
But Mr Cameron told Today: "The principle, I think, should be this: if you are fundamentally changing the relationship between Britain and Europe, then you should be having a referendum. If we had an in/out referendum tomorrow or very shortly, I don't think that would be the right answer, for the simple reason I think we would be giving people a false choice, because right now there are a lot of people who are saying I would like to be in Europe but I'm not happy with every aspect of the relationship so I want to change."
Amid claims from some in his party that it might be in Britain's interests to leave the EU, the Prime Minister said: "I'm in favour of our membership of the European Union and I'm optimistic and confident that we can achieve changes in the European Union to make sure that Britain feels more comfortable with our relationship with Europe. I'm confident we can do that."
Reports that Mr Cameron was planning a major speech on Europe date back to September last year, and the PM himself joked that he was taking a "tantric" approach by delaying it so long. It had been thought it would finally be delivered next Wednesday, but reports suggest that plan was changed to avoid clashing with German and French celebrations on January 22 of the 50th anniversary of the 1963 Elysee Treaty sealing post-war reconciliation.
After the announcement of the date on the Number 10 Twitter feed, Labour denounced the handling of the issue as a "shambles". Shadow foreign secretary Douglas Alexander said: "When it comes to David Cameron's speech on Europe, he can't seem to decide on a date, let alone a policy. It's not so much a Number 10 strategy as a Number 10 shambles."
European Commission vice-president Joaquin Almunia has warned that it would be a "disaster" for the UK if it withdrew from the EU. He told the BBC's Hardtalk programme that renegotiating the UK's terms of membership would be "extremely difficult", adding: "If you want to be in a club and you want to be an important member of the club - as I think the UK should be - you cannot be influential and relevant saying, 'I don't want this, I don't want that'."