Exclusive: The Liberal Democrats are expecting to hoover up anti-Brexit votes and finish second in Thursday's by-election in Lewisham East.

Canvassing seen by Business Insider suggests Sir Vince Cable's party is going to leapfrog the Conservatives into second place and win a significant chunk of Labour voters.

"The anti-Brexit message has got through to people," one Lib Dem source said.

Labour candidate Janet Daby is expected to win the seat, with Lib Dem Lucy Salek and Conservative Ross Archer battling to be runner-up.

LONDON — The Liberal Democrats are expecting to hoover up anti-Brexit voters in the Lewisham East by-election, leapfrogging the Conservatives into second place, according to party insiders and canvas returns seen by Business Insider.

On Thursday, Lewisham East will vote on who should replace Heidi Alexander, who resigned as the Labour MP for the southeast London constituency last month to work for London Mayor Sadiq Khan.

Labour won 67.9% of the vote in Remain-voting Lewisham East at last year's general election. The Conservatives came second with 23%, while the Liberal Democrats were a distant third, receiving just 4% of the vote.

However, sources within the Liberal Democrats have told BI they are on course for a "strong" second-place on Thursday, in what would be a collapse of sorts for Theresa May's Conservatives and what Lib Dems hope will be seen as a "message to Jeremy Corbyn to listen to his supporters on Brexit." Labour also expects the Lib Dems to be their closest rivals on the day.

Nearly 70% of people in Lewisham voted to stay in the European Union in 2016. Labour's candidate, Janet Daby, is a vocal supporter of staying in the single market and customs union. However, the Labour leadership's reluctance to embrace a softer Brexit could see voters switch to Sir Vince Cable's Lib Dems, Lib Dems are predicting.

Corbyn's Labour has a Brexit policy of a customs union plus a new "single market deal" with the EU but is being urged to go further by its pro-EU MPs and supporters. For example, Corbyn will not tell MPs to vote in favour of keeping Britain in the single market via the European Economic Area — i.e the Norway model — in a vote this week.

In canvassing seen by BI, the Lib Dems are set to finish second place with 25% of this vote, up 21 points on last year's election. Labour will retain the seat but with a vote share 19 points down on last year. The Tories will drop 7 points to third.

The Lib Dems believe that voters are going to punish the government for its handling of Brexit and Labour for being too timid in its opposition to it.

"They don't like the absence of an opposition to Labour in Lewisham and feel taken for granted," one Lib Dem source said.

Another said: "We think we could end up in a strong second. It looks like the Conservatives haven't done much, Labour has been quite very quiet and the anti-Brexit message has got through to people.

"What you could see is this being a warning to Corbyn that he needs to start taking a real anti-Brexit stance."

Heidi Alexander, former MP for Lewisham East. Reuters/Neil Hall

These numbers are supposedly based on responses from more 20,000 people over the weekend. The number of people intending to vote for the Lib Dems has increased week-on-week since a by-election was triggered, the figures suggest.

Some Lib Dems BI spoke to threw doubt on the accuracy of the figures, specifically the size of the Lib Dem's predicted vote share. They have doubts over turnout and flagged the party's patchy record when it comes to forecasting for elections and by-elections.

"If that's internal [research], I'd be careful. 21% seems very high, one well-placed source said.

However, all insiders who BI has spoken to believe Cable's party is likely to leapfrog the party into second place.

"Not surprised by second place. That seems fair," one Lib Dem insider said. A Labour source on the ground in Lewisham described the Lib Dems' projected vote share as "a little high" but said, "the numbers seem right."

Labour's Daby is up against Lib Dem candidate Lucy Salek, Tory Ross Archer, and 11 other candidates. The by-election will take place on Thursday, after MPs vote on a number of crucial votes to the EU Withdrawal Bill. These include an amendment to instruct Prime Minister May to keep Britain in a customs union and one which seeks to give Parliament the power to decide what should happen if it rejects the Withdrawal Agreement later this year.