Britain's anti-European Union UK Independence Party (UKIP) won its second seat in parliament on Friday, in a by-election that could signal major upheaval in a general election in six months' time.
Mark Reckless was re-elected with 42 percent of the vote in Rochester and Strood in southeast England, after defecting in September from Prime Minister David Cameron's Conservative Party to UKIP, which wants strict quotas on immigration.
Furious campaigning by the Conservatives failed to stem the UKIP advance and their candidate lagged behind on 35 percent, a blow to Cameron, who had vowed to "throw everything" at Thursday's by-election battle and visited the constituency himself five times to campaign.
Reacting to the defeat, the prime minister vowed he was "absolutely determined to win" the seat back in next year's national vote, but UKIP leader Nigel Farage insisted he was "absolutely confident" the seat would remain within his camp.
In his acceptance speech, Reckless made an appeal to all voters to make UKIP the kingmaker at the general election in May 2015 in the increasingly likely outcome of a hung parliament.
"Whatever constituency you live in, whatever your former party allegiance, think about what it would mean to have a bloc of UKIP MPs at Westminster large enough to hold the balance of power," Reckless said.
"If you believe that the world is bigger than Europe, if you believe in an independent Britain, then come with us and we will give you back your country."
Farage hailed a "huge, huge victory".
"They (the Conservatives) threw the kitchen sink at it, but despite their boasts, we have beaten the ruling party of the day in this life and death struggle," he told Sky News.
- 'All bets are off' -
It is the second seat snatched by UKIP after another Conservative defector, Douglas Carswell, won UKIP its first elected seat in the national parliament in a September by-election in Clacton.
Speculation over further defections to UKIP swirled after Reckless suggested two more Conservative lawmakers could switch -- an idea quickly dismissed by senior Conservative politicians.
Cameron has already promised a referendum on Britain's EU membership if his party wins next year's general election and has taken a harder stance on immigration in a bid to reassure wavering voters.
Experts said the latest vote result could prove a key moment in the history of British politics, proving UKIP can attract a broad range types of voters.
"UKIP was not supposed to win this by-election," said Matthew Goodwin, politics professor at Nottingham University.
The growing support for UKIP is likely to make it harder for either the centre-right Conservative Party or the centre-left Labour Party to win an outright majority in what is set to be closely-fought elections in May.
"All bets are off for the general election next year, literally anything could happen," said Farage.
But some commentators questioned whether Thursday's by-election might be a protest vote that could wane as the general election approaches.
"I think what you're broadly seeing is the voters using by-elections as a means of expressing their discontent at all the mainstream parties," said political commentator and columnist for the Daily Telegraph Dan Hodges.
"It's also important to remember that UKIP haven't in fact won a by-election: they've, if you like, appropriated seats via MPs who have crossed the floor from the Tories."
- Quit after condescending tweet -
National opinion polls show Labour slightly ahead of the Conservatives, each with roughly a third of the vote, and UKIP's support steadily rising to about 16 percent.
The by-election results showed the traditional parties losing large chunks of support to insurgent parties on the left and right, mirroring trends across Europe and leaving pollsters puzzled about the possible make-up of the next parliament, and who will form the government.
"I've never been as uncertain this close to a general election how these things are going to play out," said Peter Kellner, president of pollster YouGov.
Reflecting anger at all main parties, seen as out of touch by many voters, the by-election also caused unexpected embarrassment for the Labour Party, which is struggling to convince working-class voters that it still has their interests at heart.
Senior Labour lawmaker Emily Thornberry was forced to resign from the shadow cabinet after posting a Tweet seen as condescending towards blue collar voters, showing a house festooned with English flags and the caption "Image from #Rochester".