The Tories will "categorically" not enter any electoral pact with the UK Independence Party, party chairman Grant Shapps has insisted, after the idea was proposed by a senior colleague.
He poured cold water on a call from a party vice chairman Michael Fabricant for an in/out referendum on Britain's membership of the European Union to be offered in return for the eurosceptic party not fighting marginal constituencies.
However, in contrast to David Cameron, Mr Shapps indicated such a referendum could be on the cards if Britain was unable to successfully negotiate a new relationship with the EU.
In a report for Mr Cameron, Mr Fabricant, a vice chair for campaigning, suggested a deal with Ukip could help the Tories win at least 20 seats by stemming "the continued haemorrhage" of votes to Ukip. "We cannot ignore the maths," he said, warning that Ukip had become "a significant contributory factor in costing the Conservative Party victories in marginal seats".
It was seized on by Eurosceptic backbenchers to reinforce their push for voters to be offered the chance to quit the EU amid concern about the number of Tory voters switching allegiance.
In the Commons, the Prime Minister again re-iterated his opposition to an in/out referendum although he indicated he would consider a popular vote on a "new settlement" with the EU in the wake of the eurozone crisis. "The problem with an in/out referendum is that both of the options are not really what I would want or I think the British people want. I don't think keeping our membership as it is is acceptable. Nor do I think walking away from Europe would be a good idea," he said.
"My view is that Britain should be looking for a different and better settlement between Britain and the European Union. When we have achieved that new settlement we should receive fresh consent for that. Of course, that could include a referendum."
A Conservative Party spokesman said Mr Fabricant "does a great job campaigning in by-elections, but he doesn't speak for the party on this issue".
Mr Shapps meanwhile sought to play down the significance of recent mid-term successes for Ukip in opinion polls and by-elections. "I want to win the next election outright of course for the Conservatives so that we have an outright majority and we don't have to be in coalition," he told BBC Radio 4's The World at One. "But I want to do that with Conservative candidates fighting and winning on their own ground and on their own terms and that is exactly what we are going to do. So I can categorically rule out any form of electoral pact with Ukip or anyone else." However he did not rule out the possibility of a future in/out poll if efforts to secure change failed.
Ukip leader Nigel Farage insisted there could be "no deals with the Tories", blaming in part the Prime Minister's 2006 claim that some of his party's members were "closet racists".