The UK Independence party has a virtually unassailable 44-point lead in Clacton over the Tories, as the parties go head-to-head in a by-election following the defection of Tory MP Douglas Caswell to Nigel Farage's Eurosceptic party.

Ukip, which did not field a candidate in 2010, is on 64%, with the Conservatives on 20% (down 33% since the general election), Labour on 13% (down 12%) and the Lib Dems on 2% (down 11%), reports the Mail on Sunday.

Of those polled, 57% of Ukip supporters say they are voting that way because they "like Ukip", in a sign of the incumbent's popularity 34% said it was because they "like Carswell", and 9% who say they are casting a protest vote.

Immigration is by far the main concern for Ukip voters (57%), followed by the EU (13%) and the cost of living (6%).

"Expect these figures to be cited by those who argue that the former Tory MP has cynically jumped ship in order to avoid defeat next year," commented the New Statesman's political editor George Eaton.

It is believed that Carswell's popularity in Clacton will help Ukip cement its success.

The win would give Ukip its first MP.

Some predict that other Tory MPs could defect as the general election looms, in order to protect their seat in constituencies which have polled in favour of Ukip.