Rochester and Strood: UKIP hopes for second elected MP
Votes are being counted in the Rochester and Strood by-election where the UK Independence Party is hoping to gain its second elected Westminster MP.
Turnout was just over 50% in the contest, triggered by MP Mark Reckless's defection from the Conservatives to UKIP.
Polls suggest he is on course to win the seat for the party, which campaigns for the UK to leave the European Union.
The Conservatives hope to keep the seat and break UKIP's momentum.
BBC political editor Nick Robinson said: "No-one in the Tory side is predicting that they are going to win, everyone on the UKIP side are predicting that they will."
UKIP were "downgrading expectations" by suggesting the outcome could be close, he said, adding: "There's no doubt what the final outcome will be, it's just a question of how many votes are in it."
Earlier, shadow attorney general Emily Thornberry resigned from the Labour front bench after being accused of "sneering at the electorate" by tweeting a photograph of a Rochester house displaying England flags and with a white van parked outside.
Former Conservative Douglas Carswell, a close friend of Mr Reckless, won the Clacton by-election last month, having also defected to UKIP.
Arriving at the count in Gillingham, Mr Reckless said he was not taking anything for granted.
When he arrived, Mr Farage said he was "hoping" for a UKIP win, predicting this would be "pretty bad news" for Prime Minister David Cameron.
There have been rumours that other Conservative backbenchers are considering leaving the party for UKIP, six months before the general election.
But Conservative MP Stewart Jackson said: "Any Tory MP who defects to UKIP on the basis of the result in Rochester and Strood - whatever it is - would frankly be completely insane."
Mr Reckless announced his defection on the eve of the Conservative Party conference in September - Labour described it as a "hammer blow" for David Cameron's authority over his own party, particularly those on the right.
Mr Cameron says the contest is a "two horse race" between his party and UKIP - and has visited the constituency five times to support the Conservatives' candidate Kelly Tolhurst, a local councillor selected for the party in an open postal primary.
Naushabah Khan for Labour, Geoff Juby for the Lib Dems and Clive Gregory of the Green Party were among 13 candidates standing for the seat.
How unusual is Rochester and Strood?
By Ed Lowther, BBC News data journalist
As UKIP marched towards by-election victory in Clacton, analysts sought to explain its appeal with reference to the seat's unusual demographics. Does the same reasoning work in Rochester and Strood?
- Clacton had the second highest proportion of over-65-year-olds in England and Wales.
- Rochester and Strood, by contrast, has a much younger population, with fewer over-65s than the national average.
- Both seats have higher unemployment rates than the national average.
- But in Rochester and Strood's case the difference is marginal.
Labour came second in the constituency in 2010. Leader Ed Miliband will want to reassure his party that it is not also in danger of losing seats to UKIP, which came a close second in another by-election in Heywood and Middleton in October.
Mr Reckless won the Kent seat for the Conservatives in 2010 with 49.2% of the vote. Labour came second with 28.5%, followed by the Lib Dems, the English Democrats and Green Party. The result is due early on Friday.
Full details of the results as they come in - and the reaction - will be on the BBC News website and the BBC News Channel through the night.
There is a by-election special on BBC One with Andrew Neil.
In Rochester and Strood the candidates, in alphabetical order, are:
Barker, Mike - Independent
Challis, Christopher - Independent
Davidson, Hairy Knorm - Official Monster Raving Loony Party
Fransen, Jayda - Britain First
Goldsbrough, Stephen William - Independent
Gregory, Clive - Green Party
Juby, Geoff - Liberal Democrats
Khan, Naushabah - Labour
Long, Nick - People Before Profit
Osborn, Dave - Patriotic Socialist Party
Reckless, Mark - UK Independence Party
Rose, Charlotte - Independent
Tolhurst, Kelly - Conservative