The battle for Birmingham Erdington looked set to go down to the wire as Labour claimed bad weather and low turnout meant the by-election race would be "really tight".
The vote in the West Midlands seat was triggered after well-liked Labour frontbencher Jack Dromey died suddenly in January.
If victorious, Labour's Paulette Hamilton will be the city's first black MP but she has faced a strong challenge from fellow councillor Robert Alden, who leads the Tory group on Birmingham City Council.
A Labour source told the Mirror: “The weather’s absolutely grim and turnout is looking extremely low. It’s going to be really tight.”
Mr Dromey held the seat at the 2019 general election - when Labour suffered its worst defeat since 1935 - with a majority of 3,601, but the constituency had the fifth lowest turnout in the country at 53.3%.
Activists were concerned that the seat's historical low turnout would be compounded by a day of rain.
One Labour MP campaigning on Wednesday said it was "honestly hard to say" what would happen. They added: "No love for the Tories, but turnout is very low and the weather is rotten. I’m not sure anyone could have a real sense of how it will play out."
Anything but a Labour victory would be seen as a major upset for Keir Starmer's party, given Boris Johnson's Tories plummeted in the polls in the wake of the Partygate scandal.
