Nigel Farage's UKIP has been given a boost in a new poll which puts support for the party at 20%, four points higher than last month.
It comes after he was said to have come out top in the head-to-head clashes with Liberal Democrat leader Nick Clegg over Britain's future in Europe.
It is UKIP's highest rating in a ComRes survey, while the Conservatives have scored their lowest rating so far this year at 29%, down three.
In the new poll for the Independent on Sunday and Sunday Mirror, the Tories are six points behind Labour which are unchanged on 35%.
The Liberal Democrats have fallen to just 7% - a dip of two points and their lowest since they went into coalition with the Conservatives in 2010.
Mr Farage's personal rating also appears to have received a boost after the recent debates.
The UKIP chief has enjoyed a seven-point bounce since February, taking him to 27%.
Deputy Prime Minister Mr Clegg is down one point to 12%, while Prime Minister David Cameron has dropped four points to 27% and Labour leader Ed Miliband is down two points to 20%.
Mr Cameron's rating may have been dented by his handling of the expenses furore involving former culture secretary Maria Miller.
Some 62% of voters believe he showed a serious lack of leadership in the way he dealt with the case.
Most voters, 62%, believe constituents should be able to force sitting MPs to defend their seat in a by-election if enough people sign a petition demanding it, the research found.
Sky News Political Correspondent Anushka Asthana said: "We're only a few weeks out from the European elections and UKIP are kind of what the Lib Dems used to be - the party that stands against the establishment.
"They used to just talk about Europe - now it is local, popular issues such as HS2.
"The Conservatives are really badly hit by UKIP rising because that tends to split the right.
"And pollsters say if UKIP get anything over eight points in a general election they would split the right and would stop David Cameron from winning an overall majority.
"So he will be very worried about that."
Meanwhile, a Opinium poll for the Observer newspaper puts the Conservatives on 30%, Labour 36%, Lib Dems 7% and UKIP on 18%.