Britain's anti-EU UK Independence Party won its first seat in the House of Commons Friday, sending jitters through Prime Minister David Cameron's Conservatives seven months before what is likely to be a tight general election.
Conservative defector Douglas Carswell's victory in the seaside town of Clacton came as UKIP also narrowly lost out on a shock victory in a second by-election on Thursday in Heywood and Middleton, traditionally a stronghold of the main opposition Labour party.
Support for UKIP, which wants Britain to leave the European Union and to restrict immigration severely, has soared in the last two years amid increasing disenchantment with mainstream political parties.
The party once dismissed by Cameron as "fruitcakes, loonies and closet racists" came top in May's European elections.
Its leader Nigel Farage now claims it could hold the balance of power if next May's general election produces no overall winner.
"Something big is happening here," Farage told BBC television Friday.
"People want change, they have had enough of career politicians of the three parties who don't understand the problems they face in their everyday lives."
UKIP's support for leaving the EU is central to its appeal. While Cameron has promised a referendum on leaving the EU if he is re-elected with a majority next year, he is opposed to the move if he can renegotiate Britain's terms of membership.
Carswell turned his previous majority of 12,068 for the centre-right Conservatives in 2010 into one of 12,404 for UKIP this time around, winning 60 percent of the votes cast in Clacton, southeast England.
In Heywood and Middleton, part of Greater Manchester in northwest England, centre-left Labour slumped from a majority of nearly 6,000 in 2010 to one of just 617, with UKIP in second place.
The result piles further pressure on Labour leader Ed Miliband.
His party is only narrowly ahead in most opinion polls while his personal popularity ratings lag well behind those of Cameron and Farage amid suggestions from experts that his geeky persona fails to connect with many voters.
"If Ed Miliband does not broaden the Labour coalition to better include working class opinion then we cannot win a majority government," Labour MP John Mann wrote on Twitter after the result.
Conservative chairman Grant Shapps toured TV studios early Friday telling Conservative voters considering a switch to UKIP that a vote for the europhobes could lead to a Labour government.
If people vote UKIP, "it's not going to be Nigel Farage in Downing Street, it's going to be Ed Miliband," he told Sky News.
A by-election after the defection of another Conservative MP, Mark Reckless, is due to be held in Rochester and Strood, south of Clacton, in November.
While the by-election results have handed the party momentum, Farage would not be drawn on whether UKIP had further defectors lined up.
Some experts said the results indicated that next year's election would be a fight unlike any other in Britain, where three parties -- the Conservatives, Labour and the centrist Liberal Democrats -- have traditionally dominated politics.
"We have to accept that the battle in England for May 2015 is not going to be a battle between simply three parties; it's going to be a battle between at least four parties," elections expert Professor John Curtice of Strathclyde University told the BBC.
Others suggested that UKIP's by-election successes could be short-lived.
"British political history is riddled with by-elections that were supposed to break the mould -- only for constituencies to return to their traditional parties like shamefaced husbands slipping home after a dirty weekend in Clacton with the secretary," historian Tim Stanley wrote in the Daily Telegraph.
In his Clacton acceptance speech, Carswell urged UKIP to broaden its appeal to make it a truly national party ahead of the vote.
"To my new party I offer these thoughts: humility when we win, modesty when we are proved right," he said.
"If we speak with passion, let it always be tempered by compassion. We must be a party for all Britain and all Britons: first and second generation as much as every other. Our strength must lie in our breadth."