The UK Independence Party is closing in on Labour as campaigning for next week's European elections reaches its final days, polls suggest.
The fiercely Eurosceptic party believes it could humiliate the Government in the wake of the expenses scandal by coming second behind the Conservatives in Thursday's vote.
Such a disastrous result would intensify pressure on Gordon Brown, triggering fresh speculation over a Cabinet-level attempt to topple him.
A YouGov poll of people who said they were certain to vote put the Conservatives on 26 per cent, Labour on 21 per cent, Ukip on 16 per cent and the Liberal Democrats on 14 per cent.
Ominously for Labour, Ukip is in second place among over-55s, the group most likely to vote, on 22 per cent. The Tories were on 24 per cent, Labour on 18 per cent and the Lib Dems 16 per cent.
An internal ComRes poll for Ukip is also understood to have put the party within "striking distance" of Labour with 18 to 19 per cent support among electors "certain" to vote on Thursday.
Labour strategists – already braced before the expenses storm for a battering in the election – fear they will be worst affected of the major parties by the revelations over the last three weeks. The outcome could be further complicated by a likely low turnout which could benefit smaller parties such as Ukip, the Greens and the British National Party.
In 2004 Ukip came third in the popular vote, ahead of the Lib Dems, with 16 per cent support. Ukip had 12 Euro MPs elected. To repeat that performance would amount to a remarkable recovery by the party which has suffered bouts of internal wrangling.
A party spokesman said it was "possible" UKIP would overhaul Labour next week. But he added: "It is a bit too early to call it."
A Populus poll in The Times today had 82 per cent of respondents welcoming the idea of “recalling” MPs between elections if they had broken parliamentary rules.