The Lib Dems are anticipating a Labour victory in Mid Bedfordshire, in what would be one of the biggest by-election defeats in British history.
A party source said: “We think we’ve doubled our share of the vote, and taken enough Conservative votes in the villages to help Labour get over the line.
“It’s not the progressive vote that split tonight, it’s the Conservative vote as thousands of lifelong Tories switched to the Lib Dems.”
Reform UK’s candidate, Dave Holland, also reckons Labour might have pipped the Tories to the post, with the Lib Dems taking third place.
He told The Telegraph he thinks Sir Keir Starmer’s party “look slightly ahead at the minute”.
“On the door, no one had a good word to say about the Tories,” he said.
As the biggest ever numerical majority to be overturned in a UK by-election, such a result would come as a huge blow to Rishi Sunak at a critical juncture for the Conservative Party.
Some 44 per cent of voters turned out to pick Mid Bedfordshire’s next MP, broadly in line with the figure for the last three by-elections in England.
Counting is now under way in Chicksands, with a result expected in the early hours.