William Hague has appealed to wavering Conservative voters attracted to Nigel Farage's Ukip to stick with the Tories - because they are the "only" party capable of delivering an in-out referendum.
The foreign secretary's comments came at the end of a day in which Farage's presence on the fringes of the Conservative party conference in Manchester have left some Tory activists questioning their loyalty.
Earlier Farage proposed giving party associations the power to arrange localised pacts between the two parties - in areas where the eurosceptic vote is in danger of being split, allowing Labour to win the constituency.
That proposal was roundly rejected by chancellor George Osborne, who made clear the Conservative party leadership would not accept any deal at the national level.
Now Hague has reinforced that point by arguing that only David Cameron in Downing Street will deliver the in-out referendum promised by the Tories.
"I think of it as a pact with the voters of other parties," he said at a fringe event in response to a question from Politics.co.uk.
"That is what we've often done in the history of the Conservative party, with aspirational Labour parties in the past and I hope again in the future.
"Someone who would otherwise vote Conservative but is contemplating voting for Ukip at the next general election, should vote Conservative if they want a referendum," he added.
"It is therefore a straight choice. I think we have to put that choice correctly and eloquently to the voters rather than make a pact with other parties."
The Conservatives face a severe threat from Ukip which Farage predicts will see the Tories win 2014's European elections outright.
Hague accepted changes like the 'referendum lock' - a coalition achievement originally put forward by Cameron in 2009 - would motivate voters.
"Fundamental change in the EU means changing the architecture, changing the rules," the foreign secretary added.,
"Those things sound quite abstract but they would make a big difference. You are worried you're not going to be able to get somebody with their family in the background and the dinner on the cooker on the evening of the elections to go and vote with those things.
"It will be very important in the period before the election to demonstrate the sort of things that will help so we have that narrative."
He pledged to provide Conservative activists with more details about the kinds of changes Britain will be calling for in the renegotiation with Brussels that will precede the in-out referendum.
Hague said: "We have to take this message to the country: only the Conservatives have this record of delivering for Britain in Europe. Only the Conservatives can deliver the change and the choice that Britain want."