Ukip's strong performance in traditional working-class constituencies could see it snatch a number of Labour seats at next year's general election, according to analysis by a professor of politics.
With the May 2015 poll expected to rest on a knife edge, the analysis was being seen as a warning to Ed Miliband that he cannot afford to be complacent about the ability of Nigel Farage's party to attract votes from the left as well as the right.
Labour seats which could be vulnerable to Ukip include shadow women's minister Gloria de Piero's Ashfield, Nottinghamshire, constituency, retiring veteran Austin Mitchell's Great Grimsby, as well as Rotherham - where Ukip came second in a recent by-election - Plymouth North View and Dudley North, Matthew Goodwin of the University of Nottingham found.
Dr Goodwin identified the seats at risk by looking at census data as well as results in elections dating from 2010 to this year's European Parliament and local polls, taking into account the "Ukip plus vote" of people who backed other Eurosceptic parties in the Euro-elections.
He told The Independent: "Labour is divided about how seriously it should take Ukip. Those in Ed Miliband's team who cling to the misguided view that Nigel Farage's party is not a threat should look at the Labour seats that are most vulnerable to this insurgency.
"The Labour seats that emerge as top prospects for Ukip share several features; they are typically over 95% white British; have large numbers of pensioners and voters with only GCSE or no qualifications; and in 2014 saw Ukip win the popular vote. But it is the local political context that will be crucial at next year's first-past-the-post election."
While many Labour seats have "ideal conditions" for a Ukip challenge, they have healthy majorities which will be difficult to overturn, particularly where there are well-embedded incumbent MPs with experience of fighting off the British National Party, like Stoke's Joan Walley, said Dr Goodwin.
He added: "Ukip need seats that not only have lots of receptive voters but also favourable political conditions where the vote is split across the main parties, making a third party insurgency possible."