The polls have closed in the Sleaford and North Hykeham by-election, called after its Conservative MP quit.
UKIP have been campaigning hard in the Lincolnshire seat, an area which voted heavily to leave the European Union.
The seat has returned Conservative MPs since it was formed in 1997 - Tory Stephen Phillips had a majority of more than 24,000 votes in 2015.
He resigned in November, citing "irreconcilable policy differences" with the government.
Although he backed leaving the EU, he had since been critical of the government's approach to Brexit.
Mr Phillips won with 56.2% of the vote in 2015, Labour came second with 17.3% with UKIP a close third on 15.7%.
But UKIP has been throwing its weight into the by-election campaign, hoping to bring about a similar upset to last week's Richmond Park by-election, which saw the Lib Dems overturn a 23,000 Conservative majority.
UKIP will be hoping to capitalise on Eurosceptic feeling in Sleaford and North Hykeham, situated in Lincolnshire, west of Boston - which had the highest majority of Brexit voters in Britain.
UKIP's former leader Nigel Farage and his successor Paul Nuttall have been on the campaign trail in the constituency, supporting their candidate Victoria Ayling - a former Conservative who stood for the Tories in Great Grimsby in 2010 - running Labour a close second.
Conservative candidate Dr Caroline Johnson, a consultant paediatrician, stood unsuccessfully for Scunthorpe in 2010, losing to Labour. She has said she is "completely behind the government's plans for Brexit".
Jim Clarke, a refuse driver, is standing for Labour. Party leader Jeremy Corbyn has been to Sleaford to give a speech backing Mr Clarke. The party will be hoping for a strong performance after losing its deposit in last week's Richmond Park by-election. Lib Dem leader Tim Farron has been in the constituency backing his candidate, Ross Pepper, an optical assistant.
In all 10 candidates are vying for the seat. The result of the by-election is expected at about 04:00 GMT on Friday.
The full list of candidates is:
- The Iconic Arty-Pole - Monster Raving Loony Party
- Victoria Ayling - UKIP
- David Bishop - Bus-Pass Elvis Party
- Jim Clarke - Labour
- Paul Coyne - No description
- Dr Caroline Johnson - Conservatives
- Marianne Overton - Lincolnshire Independents
- Ross Pepper - Liberal Democrats
- Sarah Stock - Independent
- Mark Suffield - No description