The finding, based on a survey of 30,000 voters, goes some way to debunk claims by Nigel Farage that he is winning over disaffected working-class Labour voters as much as middle-class former Conservatives.
The survey finds that the damage to Labour had been done before 2010 while the switch to the Conservatives is still happening.
Looking at Ukip supporters in Labour seats, only 18% voted Labour in 2010, whereas 39% were Conservative voters compared with 30% having supported Labour in 2005. Only 31% were Tory supporters in that year.
Prof Geoff Evans, from Nuffield College at Oxford University, said: “British Election Survey data shows quite clearly that it’s the Conservative party who need to worry most about the threat of Ukip because those people who supported Labour have, in the main, already made the switch.
“New Labour’s move to the liberal consensus on the EU and immigration in 2001, 2005 and 2010 left many of their core voters out in the cold a long time before Ukip were around.”
Those former Labour voters who have made their way to Ukip tended to have last voted for the party in 2005, and to be working class and anti-European Union.
Evans added: “In October, Labour narrowly held Heywood and Middleton in a strongly contested byelection, which showed high levels of Ukip support in a traditionally Labour-voting constituency.
“But BES data shows how Labour had lost these voters some time ago. Most Ukip voters who had voted Labour in 2005 had not voted for them in 2010.
“Ukip support in Labour constituencies is more likely to be taken from disaffected former Labour voters, and these are far more likely to be manual workers than the middle classes that New Labour appealed to.”
The survey shows that one in five of the 10 million voters who supported the Conservatives in 2010 is on the verge of defecting to Ukip.
It suggests that the rise of Ukip will damage the Tories significantly more than Labour, which is expected to lose 500,000 voters to Farage’s party, and the Lib Dems, who are expected to lose 700,000 voters.
A further paper from Prof Jane Green of Manchester University produces data to debunk the idea that Ukip is attracting the politically disengaged, despite Farage’s claims to be appealing to long-term non-voters.
She found Ukip was gaining the support of only 1.4% of the politically disengaged, while the Conservatives were winning 2.2% and Labour 2.7%. Ukip was, however, doing well among politically engaged but disillusioned voters who may have voted for other parties in the past.
“Political distrust and disaffection is an important part of Ukip’s success. However, to assume that Ukip is outperforming all other parties in reaching those disengaged from politics is either premature or simply incorrect,” she said.
The figures will be used by the Conservatives as further evidence that voting for Ukip will only make a Labour government more likely.
They also suggest that Ukip is likely to capitalise on its success in two byelections this year and win up to 19% of the vote.
The claims were immediately challenged by Rob Ford, an academic that has specialised in studying Ukip. He has repeatedly argued Ukip represents a threat to Labour. He said the “claim that any support Labour might lose to Ukip had gone already in 2010 is hard to square with other evidence”.
He also said it ignored the counterfactual that Ukip’s presence has prevented some of these ex-Labour supporters returning to Labour.
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