Just last year, Conservative Party grandee Kenneth Clarke described them as a "collection of clowns" – yet now they represent the greatest electoral challenge to the three main U.K. political parties for decades.
The U.K. Independence Party (UKIP) - the closest the U.K. has to the U.S. tea party - has emerged from the fringes to the limelight, winning its first seat in the U.K. parliament in a by-election on Thursday.
UKIP candidate Douglas Carswell won a by-election in Clacton, south east England by a majority of 12,404 to become the party's first member of parliament.
The election was triggered by incumbent member of Parliament (MP) Douglas Carswell's defection from the Conservative Party to UKIP. The party briefly had one MP in 2008, when then-Conservative MP Bob Spink defected.
Clacton - with its working class, elderly, white and economically left-behind population - was already identified as one of the constituencies most likely to vote UKIP in May's general election by Matthew Goodwin and Rob Ford, authors of "Revolt on the Right" and two of the best-known experts on the party.
'Gone by Christmas'
A by-election further north may actually be more concerning for the main political parties. In Heywood and Middleton, a safe Labour seat to date, the death of the local MP has triggered a vote.
Labour candidate Liz McInnes won the vote by a margin of 617 -- a far cry from a 5,971 majority at the 2010 general election. The results suggest that UKIP has made significant inroads there and gone beyond attracting only right-wing Conservatives, but also left-wing voters, who feel threatened by cheap labor from immigrants.
"UKIP supporters are very pessimistic on the economy," John Curtice, professor of politics at the University of Strathclyde, told CNBC.
"The improvement in the economy hasn't trickled down to the older working-class, and that's UKIP's constituency."
Referendum on EU membership
UKIP member of European Parliament (MEP) Patrick O'Flynn wrote that if UKIP's candidate comes "a good second" in Heywood and Middleton, "my party will have shown that across the north of England it is replacing the Tories and Lib Dems as the main opposition to Labour," writing in the right-wing U.K. tabloid the Daily Express on Wednesday.
This may pile extra pressure on Miliband to commit to a referendum on U.K. membership of the European Union, which he has so far refused to do. However, the trade unions which helped him gain his leadership of the Labour Party, and some of his own MPs, have suggested that he should make this stand, in an effort to guard against the erosion of the white working-class vote. If he does, the spectre of a "Brexit" – British exit from the EU - could have a huge impact on U.K. investments and diplomatic relations.
General election looms
This all means that next year's U.K. general election is shaping up to be the most important since Labour swept to power in 1997.
Cameron has warned traditionally-Conservative voters tempted by UKIP that they could be "going to bed with Nigel Farage and waking up with Ed Miliband", as he tries to keep those on the right wing of the party, who are thinking of casting their vote for UKIP, on board.
Politicians from the traditional parties - watching opinion polls which now regularly show UKIP winning a greater share of the vote than the Liberal Democrats, currently in a coalition government with the Conservatives - are hoping that the U.K.'s "first past the post" voting system will prevent UKIP from translating their share of the vote into seats in Parliament.
"UKIP poses a potentially big 'known unknown' going into the 2015 election, even though the probability of it winning more than a small number (i.e., single figures) of seats appears to be low given the U.K.'s 'first-past-the-post' system for parliamentary elections," Alastair Newton, senior political analyst at Nomura, pointed out in a research note.
- By CNBC's Catherine Boyle.