David Cameron 'bullish' about UK's EU renegotiations
David Cameron has told the BBC he is "upbeat, bullish and optimistic" that he will succeed in renegotiating the UK's relations with the European Union.
He said he would "absolutely" give a cast-iron guarantee that a referendum would be held by the end of 2017.
Asked if that meant he would recommend voting to leave the EU if he failed to get the changes he wanted, he said he was confident the talks would succeed.
UKIP's Nigel Farage accused Mr Cameron of "pretending" to be Eurosceptic.
Appearing on The Andrew Marr Show, Mr Cameron restated his plans to secure changes to EU powers over the UK in areas of immigration, welfare, the single market and the ability of national parliaments to over-rule Brussels.
Other EU leaders
He said the most important change was ensuring the goal of "ever-closer union", contained in the Treaty of Rome, did not apply to the UK.
"We have the referendum whether or not I have successfully negotiated," he said. "I believe I will be successful.
"Others in Europe need change to the European Union. The Eurozone needs change - it needs a banking union, it needs more fiscal union - and so we're perfectly legitimate to ask for our changes.
"You've already seen other leaders in Europe including for instance Chancellor Merkel in Germany, not rejecting out of hand what Britain is saying but saying 'yes, we can discuss these issues'."
Mr Cameron said he would not sit on the fence during the in/out referendum.
Pressed on what his stance would be if he did not succeed in negotiating the changes he hoped for, he said: "We'll be having a referendum, where I'll be recommending 'look I've got these changes, I believe we should stay in this reformed organisation'."
The Conservative leader said his message was a positive one compared with the UK Independence Party's one of wanting to leave the European Union.
He said: "I'm someone who has a very positive optimistic plan for this country - we have turned around Britain's economy.
"I'm tremendously upbeat, bullish and optimistic about what Britain can achieve in terms of our economy and also about our place in Europe.
"We have a plan and that is the most important thing in politics."
Mr Cameron repeated his pledge to quit as prime minister after the 2015 election if he could not guarantee the holding of a referendum on whether the UK should stay in the EU.
UK Independence Party leader Nigel Farage told the BBC's Sunday Politics that Mr Cameron had a problem being believed, after failing to hold a referendum on the Lisbon treaty.
Mr Farage added that Mr Cameron was "desperately pretending to be a Eurosceptic whilst at the same time saying whatever the results of all this he will campaign for Britain to remain in".
Labour's Michael Dugher said the interview had shown the Conservatives "offer no answers to the big challenges facing the country".
The shadow Cabinet Office minister said: "Families in Britain are facing a massive cost-of-living crisis, yet all Cameron offers is more reheated and vacuous banging on about Europe."