David Cameron is coming under increased pressure from senior Conservatives to stage a referendum on Europe before the next general election in an attempt to halt the rise of Nigel Farage's UK Independence Party.
Tory right-wingers are calling on the Prime Minister to hold a "mandate referendum" next spring, according to reports in The Sunday Telegraph and The Observer.
Voters would be asked whether they supported Mr Cameron's plan to negotiate a new relationship between Britain and the European Union. The new settlement could then be put to a second referendum after the general election in 2015 in line with the Prime Minister's existing proposals.
Former Tory leadership contender David Davis said that staging the first referendum to coincide with elections to the European Parliament next May would be "an absolute Ukip killer".
"It would massively reinforce the Prime Minister's negotiating hand and put some steel in his back," he told The Observer. "He would be more likely to return from any negotiation with a good deal that would mean we stay in the EU."
Another Tory supporter of the plan, backbencher David Ruffley, said Mr Cameron needed a sharper message on Europe as many people were unaware of his referendum pledge. "This would be a concrete vote showing that Conservatives are serious about getting a referendum. It is about focusing minds. This Ukip insurgency is real and it's serious," he told The Observer.
Earlier, Home Secretary Theresa May acknowledged that ministers needed to offer voters "greater certainty" that they would honour their commitment to give voters a say. I think what we need to do is to be able to show people that we will hold that referendum," she said.
But any attempt by Conservative ministers to hold a referendum in the current parliament would almost certainly run into opposition from their Liberal Democrat coalition partners. An alternative option could be for Tory ministers to support a backbench referendum bill. Even if Labour and the Lib Dems combined to defeat it, it could serve to demonstrate ministers' commitment to the principle of a referendum.
Meanwhile Ukip treasurer Stuart Wheeler suggested that the party could enter a pre-election pact with the Tories if Boris Johnson was party leader. "If Boris Johnson's policies were acceptable, which I think on the whole they would be, I would be happy to do that," he told The Sunday Times.
Many in Ukip have never forgiven Mr Cameron for his description of the party as "fruitcakes, loonies and closet racists" and Mr Wheeler's remark is likely to be seen by No 10 as mischief-making.