The Conservatives expect to retain the constituency, which was held by former Cabinet minister James Brokenshire until his death in October, despite a bruising few weeks for Boris Johnson’s administration.
The contest is taking place following damaging headlines over allegations of sleaze, claims that lockdown rules were broken in No 10 and the emergence of the Omicron coronavirus variant.
Local Tory councillor Louie French is the bookmakers’ firm favourite to win the election, with Labour acknowledging that they did not expect an upset.
Mr Brokenshire held the south-east London seat with a majority of almost 19,000 at the 2019 general election, taking 64.5% of the vote.
A string of senior Tories, including the Prime Minister, have visited the constituency during the by-election campaign, with Transport Secretary Grant Shapps among the MPs bolstering the effort to get out the vote on Thursday.
But a turnout of just 34% – 21,788 votes – suggests the by-election contest has failed to excite the public in the constituency.
Labour also had a high-profile on the streets of Old Bexley and Sidcup, with shadow chancellor Rachel Reeves and her sister and shadow solicitor general Ellie among those knocking on doors on polling day on behalf of candidate Daniel Francis.
Labour were second in 2019, with 23.5% of the vote and will hope to make inroads into the Tory vote share.
After polls closed, Ellie Reeves said: “This seat has always been a safe Tory seat and we don’t expect that to change tonight.
“However, people are increasingly fed up of the Prime Minister’s broken promises – on tax rises, social care and NHS waiting lists. Patience is wearing thin with Boris Johnson.
“We’ve run a positive campaign and people have been open to hearing Labour’s offer, but to win this was never within reach for us.”
Other candidates include Simone Reynolds for the Lib Dems, Jonathan Rooks for the Greens, and Reform Party leader Richard Tice.
The constituency has been held by the Conservatives since its inception in 1983, and in its previous incarnations since the 1950s, with former prime minister Edward Heath among those to represent it.