Nigel Farage has said Ukip 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 accused David Cameron of trying to kick the national vote, planned for 2017 if the Tories win outright next May, "into the long grass".
But the eurosceptic party would agree to a confidence and supply arrangement - where it backs or abstains on budget and legislative programme votes - with a minority Conservative government in return for a quick and full referendum.
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."
“Some time in July next year strikes me as a very good time to do this."
Such an arrangement would require Ukip to hold enough parliamentary seats to make a material difference in the division lobbies.
But Mr Farage predicted that Ukip would continue to send shock waves through the Westminster establishment.
The next by-election battle, which will be fought in Rochester and Strood, could lead to either David Cameron or Ed Miliband being ousted if their parties are given a further drubbing, he claimed.
Pressed on who he would prefer to be prime minister after the general election, Mr Farage told the programme: "Ask me that after the Rochester by-election because I think there is a possibility that one or both of those leaders may not be leading their parties in to the next general election."
Douglas Carswell, Ukip's first elected MP, has insisted the party is on course for victory in the Kent seat being fought by fellow Tory defector Mark Reckless, adding the win would "fundamentally" change British politics.
Recent polling has put the eurosceptics nine points ahead and Mr Carswell said it "felt a little bit like Clacton".
He told BBC1's Andrew Marr Show: "I think things are looking OK there. 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".
The Clacton victor likened his former party to failed music store HMV and claimed the Conservatives were now "defunct".
"The way the Tory party is retailing politics is like the way HMV retailed music. It's a defunct retail model," he said.
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.
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.
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.
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