Voters will be offered "a real way of giving consent" to Britain's ongoing membership of the European Union, David Cameron has pledged.
The prime minister gave his strongest hint yet that the next parliament will see a referendum on the UK's place in Europe on BBC1's The Andrew Marr Show this morning.
Cameron has so far resisted pressure to guarantee an in-or-out referendum under a Conservative government after the next general election.
He is expected to make his position clearer in a speech on Europe set for mid-January, but hinted at its contents when he said: "People should be in no doubt the Conservatives will be offering a real choice and a real way of giving consent to that choice."
The exact terms of such a "choice" remain unclear. Tory eurosceptics are calling on Cameron to provide a robust response to the growing threat posed by Ukip, which has registered an impressive set of recent by-election results and is expected to win 2014's European elections.
Cameron, who in opposition had described the right-wing party as "fruitcakes, loonies and closet racists", turned down an opportunity to modify those comments today, instead adding that Ukip contained "some pretty odd people".
Ukip leader Nigel Farage, speaking on Sky News' Murnaghan programme shortly afterwards, said: "If he wants to go on being rude about me and rude about Ukip well let him do it, we won't lose any sleep over it."
Farage rejected Cameron's argument that Britain was better off influencing the European single market from the inside.
"Fifty per cent of our trade is with the EU," the prime minister had argued.
"Because we have a seat at the table in the single market, we help write those rules. If we were outside... we'd have no say over the rules into the market into which we sell."
Cameron said Britain could end up like Norway, which as a member of the European Economic Area has to pay a fee to trade with EU countries.
Farage said Norway was "rich, independent, prosperous, doing well," however. He said Britain would still face "masses of legislation coming in each year" and would be negatively affected by rules allowing free movement of people.
The Ukip leader also argued the UK's diplomatic clout within Europe has been damaged by its opposition to fiscal integration measures, after Cameron's December 2011 veto.
"Britain is hated," Farage said. "We've got very little influence at all."
Cameron and chancellor George Osborne have surprised some with their enthusiasm towards the reforms now being carried out by other EU states to address the unresolved eurozone crisis.
The prime minister said: "I'm very positive about the changes they need to make, but I think it is a perfectly acceptable argument to say as you make your changes there are changes we would like to make too."
The coalition government is carrying out a 'balance of competences' review of the relationship between rules emanating from Brussels and the UK.
Cameron's speech is expected to reveal how far the current government intends to go in calling for changes as a result of its findings.
He told the Sunday Telegraph: "I will be the one, and the Conservatives will be the party, offering genuine change and a genuine choice and a real say at the next election."
Farage rejected Cameron's approach, which he dismissed as inadequate. "If this really matters, call the referendum before the next general election," he said.