Nigel Farage has described David Cameron as “somebody we can sit down and talk to” after the election in the event of a hung parliament.
The Ukip leader said his party “could never” do a deal with Ed Miliband because of his position on the EU referendum.
He told broadcasters: “He has said he will not give the British people a referendum on the great European question.”
But he added: “Mr Cameron at least has been forced into promising that, and so after the election he is somebody we can sit down and talk to.”
The remark will increase pressure on the Tories to rule out a coalition with Ukip if they fail to secure a majority on May 7.
Labour has urged the Prime Minister to “come clean” over whether he would strike a post-election deal with the party.
“Mr Cameron at least has been forced into promising that, and so after the election he is somebody we can sit down and talk to.”
Shadow health secretary Andy Burnham has written to Mr Cameron demanding that he explain what his position would be if Ukip held the balance of power.
Both Mr Cameron and Chancellor George Osborne have failed to rule out a pact.
But Conservative defence secretary Michael Fallon today excluded the possibility of “working with” Ukip in the event of a hung parliament.
He was initially asked on BBC News why the party would not rule out a coalition with Ukip, to which he replied: “Because we want a majority government.”
But asked whether the Tories would be prepared to work with Ukip if necessary, he said: “No, We have already said we are going for a majority government. We are not in the business of doing deals.”
He was then asked to imagine the scenario where no one party can form a majority government and in that instance whether the Conservatives would work with Ukip.
The Tory minister replied: “No. Look, it is our job to put over our policies.”
Asked to clarify if that was a no, he said: “That was a no. We are pointing out the dangers of going for a coalition.
“If you vote for the Conservatives, you will have David Cameron with a proper working majority ... We are working flat out for a proper majority.”
Mr Osborne was challenged directly on the issue by shadow foreign secretary Douglas Alexander after last night’s debate, but would say only that he was fighting for a Conservative majority. Mr Cameron gave the same response when questioned in a TV interview.
Mr Burnham claimed a Tory-Ukip pact was supported by large numbers of Conservative MPs and activists, and warned it would be a “poisonous proposition” which would spell the end for the NHS in its current form.
In a post-debate clash on BBC2’s Newsnight last night, Mr Alexander told the Chancellor to “look down the lens” and dismiss the suggestion.
Mr Osborne replied: “We want a majority Conservative government ... I rule in a majority Conservative government.”
Challenged to rule out a deal with Ukip while out campaigning the morning after taking on Mr Farage in the TV debate, Mr Cameron told Sky News: “We are going to win outright. That’s my aim in this election.”
Meanwhile, Tory chief whip Michael Gove slipped into German when he was asked about a possible arrangement with Ukip, saying: “Nein danke.”
In his letter, Mr Burnham said it was “clear” that Mr Cameron was preparing to do a deal with Ukip and urged him to “come clean”.
“As someone who follows rather than leads their party, you will know that a deal with Ukip is what the Tory Party wants,” wrote Mr Burnham.
“A growing number of your MPs and up to half of Conservative activists are arguing for it.
“Local Tory parties are already striking deals with Ukip up and down the country and, of course, two of your MPs have already crossed the floor.
“We know the terms of such a deal. Nigel Farage has said he would work with you in exchange for ‘a full and fair referendum to be held in 2015’. You have said you would be ‘delighted’ to offer this.
“But the real terms of a deal would see the end of the NHS as we know it.
“Your Health and Social Care Act has increased NHS privatisation, and Ukip want to go even further.”
Mr Burnham cited Mr Farage’s earlier backing for “an insurance-based system of healthcare” and said Ukip’s ex-Tory MP Douglas Carswell had called for an “open market” in healthcare contracts.
“This shared agenda will be the real basis for your decision to work together. Your deal with Ukip is a poisonous proposition that would deny working people the care they rely on from a service they cherish,” Mr Burnham told the PM. “It is now essential that you come clean about your plans.”