An anti-Brexit campaign has been forced to defend its new tactical voting tool after drawing criticism for advising voters to back the Liberal Democrats in some seats where the party trails behind Labour.

As the UK prepared for the first December election in decades, Best for Britain unveiled an online guide for pro-EU voters on the best way to thwart Boris Johnson, offering seat-by-seat recommendations on the best pro-referendum candidate to back.

However the campaign’s tactical voting tool came under fire over its recommendations in certain seats, with Labour accusing Best for Britain of offering “bogus advice” to voters.

Trials of the website show in Kensington, where Labour’s Emma Dent Coad has a 20-vote majority, voters were told to back the Liberal Democrats, who came a distant third in 2017 with 4,724 votes.

In cities of London and Westminster, Tory MP Mark Field won the seat with 18,005 votes, followed by Labour on 14,857 and the Lib Dems on 4,270 – yet voters were told Jo Swinson’s party was the most effective challenger.​

It also advised voters support the Lib Dems in Labour-Tory marginals such as Macclesfield, where the party came a distant third in 2017.

A Labour source said: “The only person who benefits from this bogus advice is Boris Johnson and the vested interests he protects.

“This false information makes Johnson’s sell-out Brexit deal more likely and its peddlers should be ashamed of themselves.

“A vote for the Lib Dems in almost every seat in the country helps put Johnson in Downing Street.”

But Best for Britain robustly defended its data and argued that it was recommending voters back Labour in some 375 seats, compared to 180 recommendations for the Liberal Democrats.

Chief executive Naomi Smith said: “Our methodology is data-driven. This data is from October 2019 and factored-in responses from 46,000 Brits.

“There’s no organisation in the UK with such sophisticated seat-by-seat data.

“Criticism based on the 2017 general election, while understandable, is therefore outdated.

“As pollster Lewis Baston said, nearly a third of voters changed parties between 2015 and 2017. And it’s pretty well accepted that a lot has changed since then.”

The row comes after the launch of the tactical voting tool at a cross-party event with ex-Tory Dominic Grieve, Labour MP Anna McMorrin and Sir Vince Cable, the former Liberal Democrat leader.

Research unveiled at the event revealed the Conservatives could win a 44-seat majority in the upcoming election, with 364 seats, compared with Labour’s 189, 23 for the Lib Dems, three for Plaid Cymru and one for the Greens.

But if 30 per cent of voters cast their ballot tactically, pro-referendum parties could strip Boris Johnson of his majority, according to a seat-by-seat analysis of 46,000 people over September and October.

In this scenario, the Tories would win 309 seats, Labour has 233, 34 for the Lib Dems, four for Plaid and one for the Greens – resulting in a 4-seat majority for pro-EU parties.

If 40 per cent of pro-EU voters cast their ballot tactically, the Conservatives would win 277 seats, Labour 255, Lib Dems 44, four for Plaid and one for the Greens – delivering a 36-seat majority for pro-referendum parties.

Independent Minds Events: get involved in the news agenda

The SNP was expected to win 52 out of Scotland’s 59 parliamentary seats in all scenarios, while the Brexit Party was not expected to win a single seat.

The research was carried out for Hope not Hate and Best for Britain by focaldata, a polling agency that specialises in understanding niche audiences using a technique called MRP (multi-level regression with poststratification) to estimate opinion to each constituency.

The MRP polling model was used to successfully predict the US election in 2016 and the hung parliament in the 2017 snap election.