The Tories will "categorically" not enter any electoral pact with the UK Independence Party, party chairman Grant Shapps insisted after the idea was proposed by a senior colleague.
He poured cold water on a call from 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.
In a report for David Cameron, Mr Fabricant, a party vice chair for campaigning, suggested the strategy could help win at least 20 seats by dealing with "the continued haemorrhage" of votes to Ukip.
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.
But Mr Shapps played down the significance of mid-term success for Ukip in opinion polls and by-elections and said an in/out referendum was not the right policy at this stage.
"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."
Ukip leader Nigel Farage retorted that there could be "no deals with the Tories: it's war", blaming in part the Prime Minister's 2006 claim that some of his party's members were "closet racists".
"Cameron's comments over the Rotherham case mean a deal's simply not possible," he said amid the controversy over children being removed from foster parents in the South Yorkshire town because of their Ukip membership.
"How on Earth could we trust him, given that he gave a cast-iron guarantee on a referendum once before?" he added, calling Mr Cameron "the real obstacle" to any pact.