Voters in the southern English town of Eastleigh went to the polls on Thursday to elect a new member of parliament in a contest that threatens serious repercussions for Britain's main parties.
The election pits Prime Minister David Cameron's Conservative Party against its junior coalition partners, the Liberal Democrats, while the anti-EU UK Independence Party (UKIP) is hoping to capitalise on voter disillusionment.
The election was sparked by the resignation of disgraced former energy minister Chris Huhne, a Liberal Democrat who has pleaded guilty to trying to avoid a speeding fine.
Nick Clegg, the embattled Deputy Prime Minister and Lib Dem leader, has called it a "two-horse race" between his party and the Conservatives, while UKIP leader Nigel Farage predicted a "big" swing to his party.
The Lib Dems have been damaged by an ongoing sex scandal surrounding the party's former chief executive Chris Rennard, and the vote looks set to cause ructions within an already strained coalition, whatever the result.
On the eve of polling, Cameron urged Conservatives to back candidate Maria Hutchings, who vowed to help "get the country back on its feet" if she won.
But senior Conservative David Davis warned that a loss for the party would place serious doubt over Cameron's leadership of the party.
"If we came third it would be a crisis," Davis told BBC television. "And if it's a close second with UKIP on our tail it will also be uncomfortable."
More than 79,000 people are eligible to vote for one of the 14 candidates, and residents have been subjected to incessant campaigning since the election was called after Huhne's resignation on February 5.Clegg visited Eastleigh on Wednesday to pledge his support for candidate Mike Thornton, saying he was on the "cusp of a great, great victory".
Addressing supporters at Lib Dems headquarters, Clegg called the race the "most exciting and closely contested by-elections" that he could remember.
Farage backed his candidate, Diane James, to "come up on the rails" and cause a major shock."If you gave me evens on us gaining more than 20 percent in this by-election I would have a very big bet," he said. "This is the campaign that has got momentum."
John O'Farrell, the candidate for the main opposition Labour party, is fighting not to finish in fourth place, and said he hoped voters would register their dissatisfaction at living standards by voting for his party.
Polls opened at 0700 GMT and close at 2200 GMT.