The UK Independence Party (Ukip) has boosted its support at the expense of the Conservatives to achieve its highest-ever rating in a ComRes survey for i's sister paper The Independent.
The anti-EU party has increased its share of the vote from 6 per cent to 9 per cent in the past month, while the Tories have slipped back by four points. The findings will heighten Tory fears that Ukip could deprive it of victory at the 2015 general election by splitting the anti-Labour vote and allowing Ed Miliband's party to regain power – even though Ukip might not win any seats itself.
Ukip, whose leader, Nigel Farage, claims it is on course to become Britain's third party, is breathing down the necks of the Liberal Democrats, who are on 10 per cent.
The rise of Ukip has helped Labour to extend its lead from seven to 10 points since the most recent ComRes survey for this newspaper at the end of November. Labour is now on 41 per cent (down one point), the Tories on 31 per cent (down four points), the Lib Dems 10 per cent (unchanged) and other parties, including Ukip, on 18 per cent (up five points). If repeated at a general election, these figures would give Labour an overall majority of 110, with the Tories losing 99 seats and the Lib Dems seeing 36 of their 57 MPs defeated.
Although Mr Farage's party is trying to woo disaffected Tories who oppose gay marriage as well as David Cameron's stance in Europe, it appears to be broadening its appeal beyond natural Conservatives.
Ukip appears to be more popular among men than women, and among 18 to 24-year-olds and those over 55. It seems to attract more support among the C2 skilled manual workers and bottom D and E social groups than people in higher social grades.
Amid growing disenchantment with the two biggest parties, Ukip is clearly benefiting from the "protest votes" that used to be garnered by the Lib Dems, but which Nick Clegg's party has struggled to pick up since entering the Coalition with the Tories in 2010.
* ComRes interviewed 1,000 adults by phone between 14 and 16 December. Data was weighted to be demographically representative of all adults in Britain, and also by past vote recall.
ComRes is a member of the British Polling Council and abides by its rules.
According to ComRes, 22 per cent who now support Ukip voted Tory in 2010, while 15 per cent backed Labour and 7 per cent the Lib Dems.