Ukip is gathering support from leftwing voters as the anti-EU party continues to widen its support base.
New poll figures show Labour's lead has dropped four points (to 34%) over the Conservatives (on 28%). The Tories and the Liberal Democrats (on 11%) have also fallen four points, according to the survey by ICM.
Support for Nigel Farage's party is up nine points, to an all-time high of 18%.
The poll findings will make disturbing reading for all the leaders of the major parties particularly in the wake of the county council elections that put Ukip on a quarter of the vote.
A slash in Labour's mid-term lead is bad news for Ed Miliband after lukewarm gains at recent English council elections.
Ukip's continuing climb in popularity is rattling the Conservative party and causing a huge headache for the PM.
In a bid to see off the threat from the right, the government has published a bill guaranteeing an EU in/out referendum. It would be put to parliament before the next general election.
The draft was an effort by Cameron to quell worries on his own backbenches by writing in to law the vote on whether Britain should leave the EU. Backbenchers have joined Farage in casting doubt on whether the PM will keep his promise to hold the vote after polling day in 2015.
Cameron's move only underlined how out of control of events he is while in coalition with the Europhile Lib Dems
The ICM survey results indicate that Tory rebel MPs such as Nadine Dorries now increasingly set the agenda on Eurpe.
Attitudes against the poor
Elsewhere, a separate survey appears to bolster the claim by rightwing rebels that they are speaking up on behalf of the nation as the centre-ground of British politics shifts rightwards.
According to the Joseph Rowntree Foundation, attitudes among Labour supporters to the poor in society are hardening.
It states there is a "general trend" of decreasing sympathy for poor people and that the majority of those polled are pulling back from their support of welfare benefits.
Increasingly, poverty is seen as the fault of the individual, not a failure of society, they say, and nearly half of Labour supporters think claimants would be more personally responsible if benefits were "less generous".
Ten percent more Labour supporters class welfare recepients as "undeserving" than a decade ago.
Julia Unwin, chief executive of the foundation, said: "The stark findings of this report highlight the increasingly tough stance people are taking against people in poverty. We appear to be tough on those experiencing poverty, but not tough on its causes."
Ukip's continual climb is eye-catching, but it remains to be seen whether Farage can convert burgeoning support in to real power.
A poll by Ipsos Mori found that fewer than one voter in 10 sees Europe as the most important issue for Britain.