Labour voters are defecting to Ukip because the party has become too "middle class", according to a former adviser to Ed Miliband.
Lord Glasman, a policy guru ennobled by Miliband, said the rise of Nigel Farage's party would hit the Opposition "in the heartlands".
In an interview with The Times, the peer said voters' concerns about immigration and welfare needed to be addressed to stop them abandoning Labour permanently.
"That is the dilemma at the heart of the party's strategy - is it possible to address these economic, political and cultural concerns when the party is becoming in many ways very middle class?" he said.
"What I mean by that is liberal and progressive in its sensibility. Ed (Miliband) is trying to address it. This is a long-term trend since 2001, in terms of the working class vote just declining quite dramatically.
"The Labour middle class vote held up (in 2010). It was the working class vote that died. These are often people who are earning, who have jobs, but they don't see Labour as representing their interests."
The comments emerged after a YouGov poll for the Sunday Times gave Ukip an outright advantage in the European election contest for the first time, with 31% support compared with 28% for Labour, and the Tories languishing in third on 19%.
The results are in line with a ComRes poll for the Sunday Mirror and Independent on Sunday last month, which put Ukip on 30%, Labour on 28%, and David Cameron's party on 21%.
The surge comes despite a series of controversies rocking the party, the latest of which saw would-be councillor William Henwood suggest comedian Lenny Henry should emigrate to a "black country".
Last week Ukip poster boy Andre Lampitt was suspended after arguing on Twitter that Islam was "evil" and Ed Miliband not British. The party has also come under heavy fire for a hard-hitting ad campaign warning of the dangers of immigration.