LONDON (AFP) - Polls opened in a by-election on Thursday that looks set to shake up British politics by securing the anti-immigration, anti-EU UK Independent Party (UKIP) its first seat in parliament.

Lawmaker Douglas Carswell defected from Prime Minister David Cameron's Conservative party to UKIP in August, and looks set to comfortably retain his seat in Clacton on the southeast English coast for his new party.

UKIP's support has been steadily growing for years and it secured the largest number of votes in European Parliament elections in May, taking 27 percent of the vote.

But it has yet to win a seat in the House of Commons - a major breakthrough that UKIP leader Nigel Farage is confident will be achieved in the early hours of Friday morning when the Clacton results come in.

"People have often said 'I like you, but I can't vote for you because you can't win'," Farage told The Times newspaper on the eve of the vote.

"The busting of that perception is very, very important." Winning its first seat would be a huge boost for UKIP ahead of the May 2015 general election, where it is hoping to win several more members of parliament.

Another Conservative MP, Mark Reckless, followed Carswell in defecting to UKIP last month although pundits suggest he will face a tougher challenge to hold onto his seat in Rochester and Strood, east of London.

Another by-election is also taking place on Thursday, in Heywood and Middleton in Manchester, sparked by the death of Jim Dobbin, the local opposition Labour MP.

UKIP is expected to make major gains in that vote although polls indicate Labour will hold on to the seat.