The recent YouGov survey indicated that UKIP is ahead of the Lib Dems. According to this measure, the Conservatives are on 31 points, Labour is on 23 points, UKIP takes 18 points and the Lib Dems are on 15.

With the expected low turnout in June, which YouGov estimates will be no higher than 39 per cent, makes a breakthrough to third place more likely. With such a result, the party which advocates a complete withdrawal from the EU could expect to make sweeping gains in the European parliament.

It would also be a blow to both the Lib Dems and the Tories who want to capitalise on the government's mid-term troubles.

With most UKIP voters coming from the Conservative ranks, Michael Howard could make few gains on the result achieved by William Hague in 1999.

"Even if it is right that the Tories are on 31 points, then that's a lead of eight points," a Labour official said in response to the poll.

However many of those questioned would vote differently in a general election, with 36 per cent backing the Conservatives, 33 per cent supporting Labour, the Lib Dems on 19 per cent and four per cent for UKIP.