Labour and the Conservatives are neck and neck in the fight for Nadine Dorries' former Mid Bedfordshire seat, according to new constituency polling.
The latest data from the traditionally safe Tory seat puts the two parties both on 20%, with the Liberal Democrats on 15%, when undecided voters are included.
When undecided voters are excluded, both Labour and the Tories are on 29%, with the Liberal Democrats at 22%.
The poll found 27% of those intending to vote were currently undecided. Commissioned by the Labour Together group and carried out by polling company Survation, it suggests a major drop-off in Conservative support in the constituency from Ms Dorries' 60% share of the vote at the 2019 general election.
But it will add to fears in Sir Keir Starmer's team that Rishi Sunak's party could still retain the seat if voters split between the Lib Dems and Labour.
The poll is based on 559 respondents, with 481 contacted by phone and 78 younger participants targeted online.
Labour figures have pointed to the polling as evidence of the need to back the party in a few weeks' time.
The Mid Bedfordshire by-election was discussed in this month's episode of Anglia Late Edition - watch an excerpt.
Josh Simons, director of Labour Together, said: "Our polling clearly shows this is a two horse race between Labour and the Conservatives. After 13 years, Britain needs the Conservatives out and Keir Starmer's Labour in.
Shadow health secretary Wes Streeting, who was campaigning in the constituencyon Saturday, said on social media: "Saw this with my own eyes in Mid Beds today: disaffected Tories are turning to Labour in huge numbers."
Lib Dem leader Sir Ed Davey was also back in the local area canvassing onSaturday. A party source said that the Lib Dems were "fired up" by the poll and said itshowed the party is "on track to win with five weeks to go".
"This is a devastating poll for Labour, who have now stalled, whilst the Liberal Democrats' vote share surges. Labour have officially hit their ceiling, whilst we begin to pull ahead in rural villages.
"This poll confirms Conservative voters are turning to us, not Labour," thesource said.
The by-election takes place on Octiober 19th. Prospective candidates have until Friday to declare that they will stand.
A list of all candidates so far can be seen here.
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know...