More than half of Ukip's support in the European elections came from disenchanted Conservative voters, a poll commissioned by Lord Ashcroft has found.

In a survey of over 4,000 people, just over half of Ukip voters said they had chosen the Conservatives in the 2010 general election. Nearly 20% had voted Lib Dem in 2010 and 15% Labour.

The poll also suggests that support for Nigel Farage may not survive into next year's general election - only 51% of Ukip voters said they would be likely to vote for the party in 2015. One in five Ukip voters is likely to vote Conservative, and one in 10 is likely to vote for Labour.

It follows an analysis of voting projections by the BBC that suggests Ukip support has dropped since last year's local elections, from 23% to 17%, throwing into question the scale of the "earthquake" predicted by Farage.

The poll by Ashcroft suggests that most people who chose Ukip did so in dissatisfaction with the main political parties. Six in 10 Ukip supporters described their vote as a general protest because they were "unhappy with all established political parties at the moment".

Before Thursday's election, Ukip leader Nigel Farage had urged voters to "send a message to Brussels – and the whole political class" by voting for his party.

"I appreciate that many of you who intend to vote Ukip next week also intend to return to the Tory fold at the general election next year," he said. But he described a Ukip vote as a "free hit against a deeply complacent and craven political establishment".

On Saturday morning, George Osborne refused to comment on whether the Conservatives would make a deal with Ukip for the general election, but said he was "not remotely complacent". Speaking on the Today programme on BBC Radio 4, he said he respected Nigel Farage, but said the Ukip leader did not have "answers to the country's future".

"We take very seriously the fact that people voted Ukip; we respect that fact. We have to listen to people who voted Ukip. We have to listen to their anger and anxiety. We need to respond to that anger and anxiety with answers."

But he warned that Ukip voters would "live with the consequences for years" if they do not switch their support to the Tories. "The job of myself and the Conservative party over the coming year is to really focus people's minds on that choice," he said. "We have to make sure people are clear about the choice at a general election, clear that only two people [the leaders of the two main parties] can be prime minister."

Commenting on the poll, which was published on the ConservativeHome website, Ashcroft said the Tories should "reconnect with these supporters".

"This will probably mean a swallowing of pride with a sincere apology for the way gay marriage was introduced and, perhaps, an admission that HS2 is not the right policy, and immigration is not under control," he said.

Shadow Cabinet Office minister Michael Dugher told Today that Labour had to "fight much harder" to tackle Ukip in the north of England. But he said the party had made "extremely good progress" and remained "on course for a majority" in the Commons.

"We are winning in the areas we need to win," he said.