Voters go to the polls in a by-election battle over the future of Heathrow
The by-election was triggered after incumbent MP Zac Goldsmith resigned as a Conservative MP, and re-ran as an independent. Goldsmith was protesting the Conservatives' decision to give the go-ahead to a third runway at Heathrow, which is a touchstone issue in a constituency underneath Heathrow's flight path.
Unusually, there's very little polling evidence available to indicate which way the vote will swing, but a BMG poll last month put Goldsmith 27% ahead of his nearest rival, the Liberal Democrats' Sarah Olney.
Bookmakers are also backing Goldsmith. These are Ladbrokes' odds as of 9 a.m. GMT (4 a.m. ET) on Thursday:
However, the New Statesman's Stephen Bush reports that the Lib Dems are privately confident of pulling off a surprise victory.
The party has campaigned on an anti-Brexit platform, and believe it may just be enough to beat Goldsmith, who campaigned for Leave - internal party polling reportedly suggests the Lib Dems are "on course to grab a narrow win," overturning the Conservatives huge majority of 23,015.
If that is the case, as pollster Mike Smithson argues, the vote today could herald a political period where Brexit is more important to voters than party allegiance.