Four opinion polls from marginal constituencies show the Conservatives falling back by a minimum of 13 points due to either Ukip moving from fourth to second place or disllusioned Tories switching to Labour.

The polls, part of a series conducted by Survation on behalf of the Ukip donor Alan Bown, suggest former Tory supporters are choosing different ways to punish David Cameron in different constituencies.

In Great Yarmouth, according to the poll, Labour is set to take the seat from the local government minister Brandon Lewis. The seat is 73rd in Labour's list of targets.

The poll shows Labour's support rising from 33% in the 2010 general election to 38%; the Toriesslip from 43% in 2010 to 28%; and the Liberal Democrats collapse from 14% to 4%.

But in a seat where there has been controversy about immigration, Ukip has surged from 5% in 2010 to second place at 30%.

The clutch of polls also shows that the education minister Edward Timpson is set to lose the Tory-held seat of Crewe and Nantwich, if the election was held today. Crewe is 86th on Labour's list of target seats. Labour is riding high at a solid 46% (up from 34% in 2010), the Tories down to 33% (46%), Ukip 11% (3%) and Lib Dems 5% (15%).

The Tories are set to retain the seat of Bognor Regis and Littlehampton, but their prime challenge will come from Ukip. The Tories are on 37% (51% in 2010), Ukip 27% (7%), Labour 20% (14% ) and the Lib Dems 11% (24%). Losing a 13,000-majority would be a huge general election blow for the Conservatives.

Further along the south coast, in Folkestone and Hythe, Ukip has again moved from fourth to second, according to the poll, but the Conservatives look set to hold the seat as a challenge from the Lib Dems evaporates. The Tories are on 35% (down from 49% in 2010), Ukip 27% (11%), Labour 21% (11%) and the Lib Dems 11% (24%).

Folkestone has been touted as the seat Ukip's leader, Nigel Farage, has been considering as a springboard to a seat in the House of Commons. On this basis, Ukip still has work to do; the results suggest Farage is more likely to plump for South Thanet, closer to the London constituencies.

But the polls will do little to calm nerves in Tory ranks as they await further ideas from strategists on how they will fight Ukip in both local and European elections.

There is also concern in Labour circles at how the Ukip surge may upset some of their own calculations as two-way marginals develop complex matrices, with votes split four ways.

Bown insisted the Survation polls were conducted independently and show that Ukip "can attract voters from all parties, based on support for their policies, not just a protest vote.… Even those 30% who did vote Conservative in 2010 may now have abandoned the party for good.

"Ukip is becoming the official opposition to Labour in parts of the Midlands and north and a real opposition to the Conservatives in many parts of the south. Voting Ukip offers the best chance to defeat incumbents in these places."