Nigel Farage has said the UK Independence Party would prop up a minority Conservative government if the Tories agreed to a swift and fair referendum on Britain’s membership of the European Union.
The Ukip leader said the party was ready to agree to a confidence and supply arrangement - where it backs or abstains on budget and legislative programme votes - if David Cameron’s party fails to secure a majority.
Mr Farage told BBC1’s Sunday Politics: “The price would be a full, free and fair referendum on our continued membership of the European Union, the opportunity to get our country back, and for that to happen quickly.”
Such an arrangement would require Ukip to hold enough parliamentary seats to make a material difference in the division lobbies.
Newly elected Ukip MP Douglas Carswell likened his former party to failed music store HMV and claimed the Conservatives were now “defunct”.
But Boris Johnson said fighting Ukip was like “fighting doppelgangers” and said he struggled to find any areas to disagree with his former colleague on.
Mr Carswell admitted that the Rochester and Strood constituency, where fellow defector Mark Reckless is standing, would be difficult to win.
Recent polling, however, has put the eurosceptics nine points ahead and Mr Carswell said it “felt a little bit like Clacton”.
“I think things are looking okay there,” he said.
“If we can win there then I think things are looking fundamentally different.”
Mr Carswell said he hoped more of his former colleagues would join him in switching allegiances but insisted the party was “not the Conservative party in exile”.
He likened the Tories to failed music store HMV. “The way the Tory party is retailing politics is like the way HMV retailed music. It’s a defunct retail model.”
Ukip has surged to 25% in the polls and some experts have claimed the soaring level of support would secure the party an astonishing 128 MPs in a general election.
Mr Carswell refused to be drawn on speculation about the number of seats the party could win.
“Let’s keep a sense of perspective. We have won a single seat. There are 300-and-something to go to get a majority.”
He added: “I don’t like bravado talk and I’m not going to use it.”
The Conservatives and Labour were left reeling after Ukip dealt both sides major blows when voters went to the polls in two by-elections on Thursday.
Mr Carswell became the first elected MP for the eurosceptics after taking Clacton with a handsome majority of 12,404 and the party was just 617 votes shy of victory in Heywood and Middleton.
Labour leader Ed Miliband is attempting to counter the threat posed by Ukip by promising reforms that would mean migrants have to “earn the right” to state benefits.
Deputy leader Harriet Harman insisted there was no “wobble” in the Labour ranks and denied a leadership change would be required despite concerns about how voters view the party and huge Ukip gains in Heywood and Middleton.
Asked if there was any chance that Labour would change its leader before the general election, Ms Harman told BBC 1’s Andrew Marr Show: “No, absolutely not. We always knew and Ed Miliband knew, the whole party knew it would be a very tough challenge after we were defeated in 2010 to be in contention to be in government and to have a Labour prime minister in 2015.”
She added: “We are not going to have a wobble or a leadership change.”
London Mayor Mr Johnson said fighting Ukip was like “fighting doppelgangers” and likened the situation to the film Face Off, starring Nicolas Cage and John Travolta where the two men swap faces.
“I was listening to Douglas and I was wondering what I really disagreed with him about and there wasn’t very much, frankly, that he said that I could disagree with,” he told Marr.
In a staggering study for the Mail on Sunday fresh off the back of the anti-Brussels party’s by-election victory in Clacton, Nigel Farage won the support of one in four voters.
The Survation poll put Labour and the Tories both on 31% while the Liberal Democrats are on 8% and experts suggest that the ratings would give Labour 253 MPs, Conservatives 187, Ukip 128, Lib Dems 11 and other parties, such as the SNP, 71.
John Curtice, professor of politics at Strathclyde University, told the Mail on Sunday: “Today’s poll suggests Ukip’s support has increased much more in the south of England outside London than it has elsewhere in the UK - by a staggering 34 points.
“If that level was recorded throughout the South, Ukip could win as many as 128 seats, with no less than 102 of them coming from the Conservatives, whose vote in the region is down 14 points.”
Private polling analysis seen by The Sunday Times, however, puts the party on course to win a more circumspect maximum of 25 MPs, although the number is still far higher than previous predictions.