Tories warn backing Ukip will keep Labour in power

The Conservatives are to warn voters that backing Ukip could allow Labour to retain power amid Tory fears that the revived anti-Europe party could deny David Cameron victory at the next election.

Ukip, which wants Britain to leave the EU, won 16 per cent of the votes at the 2004 European Parliament elections. Although it won only 2.9 per cent of the votes where it stood at the 2005 general election, analysis of the results suggests that it may have prevented the Tories winning up to 16 seats and shows it inflicted much more damage on the Tories than Labour.

John Curtice, professor of politics at Strathclyde University, said the Tories performed 1.7 percentage points worse where Ukip did well than where they did badly, while Labour dropped by only 0.6 points. This suggests that for every vote Ukip secured from Labour, it won three from the Tories.

On these assumptions, the "Ukip effect" meant that Labour held on to 11 seats that would otherwise have been lost to the Tories - Crawley, Sittingbourne and Sheppey, Harlow, Battersea, Warwick and Leamington, Medway, Gillingham, Stroud, Stourbridge, Dartford and South Thanet. The Liberal Democrats won another four seats which would otherwise have gone Tory - Romsey, Solihull, Westmorland and Lonsdale, Taunton and Eastleigh. Although it is impossible to predict Ukip's impact at the next election, the study highlights the party's potential to harm the Tories. All 16 seats will be high on the list of seats the Tories will need to win to push Labour out of office.

Since the turn of the year, Ukip has won its first foothold in parliament when two former Tory peers, Lord Pearson of Rannoch and Lord Willoughby de Broke, joined the party. Then two major Tory donors, Stuart Wheeler, who made a fortune from a City spread-betting firm, and Dixons founder Lord Kalms, said they might vote for Ukip.

Nigel Farage, Ukip's leader, has been wooing new recruits and the party is expected to announce a big donation from a former Tory backer shortly.

Despite the threat, Mr Cameron will resist pressure from some MPs and candidates in marginal seats to harden his policy on Europe in an attempt to prevent Eurosceptic voters backing Ukip rather than the Tories. He has upset some Tory MPs in recent weeks by striking a more positive tone on Europe - for example, by highlighting the role that the European Union can play in combating climate change.

"We are not going to obsessed by a few hundred votes," said a spokesman for Mr Cameron. "Ukip is trying to keep Labour in power. We are not going to pander to it. We will defeat it."

Mr Cameron stands by his attack on Ukip last year as "fruitcakes, loonies and closest racists, mostly". His aides deny that he has softened Tory policy on Europe, saying the party remains opposed to the single currency, the stalled EU constitution and the social chapter of workers' rights.

Six Tory MPs have signed a pledge saying that Britain would be "better off out" of the EU, and Ukip will not oppose them at the next election. Although the MPs will not be disciplined, Patrick McLoughlin, the Opposition's chief whip, has urged other backbenchers not to support the campaign.