A third of voters would back Ukip in the 2015 General Election, if they thought they could win, a new poll has shown.
Analysts say the exceptional growth in support for the Eurosceptic party could mean Nigel Farage's party become genuine challengers in the subsequent polls. A poll for the Observer by Opinium found 31% of voters would back the party if they could win in their constituency, 33% of Tory voters, 25% of Liberal Democrats and 18% of Labour supporters. But 40% believed it was a wasted vote.
The poll puts the Conservatives and Labour neck-and-neck on 33%, with Ukip on 18%, the Lib Dems on 6% and the Greens on 4%.
Farage is the most popular party leader, but all have negative ratings, with the Ukip leader on -1%, David Cameron on -6%, Ed Milband on -23% and Nick Clegg on -43%.
Professor Rob Ford from the University of Manchester, who co-authored a book on the party Revolt on the Right, said Ukip could be a party of opposition in subsequent elections. “If Ukip perform in line with current polling, they will secure strong second-place finishes in a wide range of seats next year, and then, like the Liberal Democrats before them, they can take their case to voters as the party of local opposition," he told the Observer.
"The large swath of the electorate willing to seriously consider the party will make this a viable option in a wide range of seats, potentially opening a wide swath of constituencies to an unprecedented challenge.”
The Tories fear that, even though the new polling shows a rise in support, a loss in the forthcoming Rochester and Strood by-election triggered by the defection to Ukip of ex-Tory Mark Reckless could mean a surge in further defections, with MPs calculating they may have more chance to win their seat if they defect.
The poll is yet to truly factor in the effect of the news on Friday that Britain must pay and extra £1.7bn to the European Union because of the economy's comparative growth.
David Cameron responded furiously to the bill from the EU yesterday, complaining that he had been ambushed at a summit of fellow leaders.
And Chancellor George Osborne is facing mounting pressure to explain what he knew and when about the shock demand.
Warning that the move risked pushing the UK closer to the exit door, the Prime Minister insisted the money would not be paid by the December 1 deadline.
But the European Commission has dismissed the objections, saying the contribution revisions were calculated by independent statisticians using a standard formula agreed by all member states. That process varies the fees charged depending on economic performance.
Britain has by far the biggest extra demand, with other countries such as the Netherlands, Italy and even crisis-hit Greece also facing paying more. France and Germany, on the other hand, have had their contributions cut.
Labour has seized on news that the Treasury was notified of the extra bill days before Cameron was informed. They also questioned why ministers failed to realise there was a potential issue earlier in the calculation process - pointing to recent upwards revisions of post-1995 Gross National Income by the UK's own statistics watchdog.
In a letter to the Chancellor, shadow chief secretary to the Treasury Chris Leslie wrote: "Following the European Council this week and the concern over the increased contribution from the UK Government to the European Union Budget you have serious questions to answer about how long the UK Government has known about the possibility of a higher surcharge for the UK."
Leslie questioned whether ministers came to parliament without giving information about the issue, and insisted full details of the calculations should be published when they obtained them from the EU.