A new tactical voting website has been criticised for advising pro-remain voters to back the Liberal Democrats in some seats where the party is way behind Labour, including key knife-edge marginals against the Conservatives.

The website, launched by the pro-remain Best for Britain organisation, suggested that voters should back the Lib Dems in Kensington, where the Labour MP Emma Dent Coad has a majority of just 20 against her Tory opponent.

The Lib Dems received just 4,724 votes there in 2017 compared with 16,313 for the Tories and 16,333 for Labour.

Likewise in Cities of London and Westminster, the website recommends voting Lib Dem, where the candidate got just 4,270 in 2017, compared with 18,005 for the successful Tory incumbent and 14,857 for Labour.

Both seats have high-profile Lib Dem defectors standing, with former Tory Sam Gyimah in Kensington and former Labour MP Chuka Umunna in Cities of London and Westminster.

Q&A What is ‘tactical voting’? Show Hide Under the first-past-the-post voting system, tactical voting is when you vote for a party that you would not normally support in order to stop another party from winning. For example, in a constituency where the result is usually tight between a party you dislike and a party you somewhat dislike, and the party you support usually comes a distant third and has no chance of winning, you might choose to lend your vote to the party you somewhat dislike. This avoids ‘“wasting” your vote on a party that cannot win the seat, and boosting the chances that the party you dislike most will lose.

Tests of the website suggested there were many examples of advice to vote Lib Dem in seats where there was little hope of victory and a vote for Jo Swinson’s party would be likely to lead to a win for the Tories.

A senior Labour party 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.”

Naomi Smith, chief executive of Best for Britain, defended the voting guide. “Our tool uses MRP analysis, which is the only model that correctly predicted the shock wins for Labour in 2017, like Canterbury and Kensington,” she said.

“Based on a sample of 46,000 people, our data is very robust and our Peterborough byelection prediction got the Labour vote share correct within 0.6%. The tool will be updated with the data for each seat in the next day or so. We expect to back Labour in over 370 seats in England and 180 or so Lib Dems. The tool only covers England and Wales.

“If 30% of anti-Brexit voters can hold their nose and back a party they wouldn’t normally, we can deny Boris Johnson a victory, and return a parliament that backs a final-say referendum on Brexit.”

Earlier, Best for Britain launched new research suggesting that tactical voting could swing a victory for pro-remain parties in the December election.

Its analysis found that Johnson would fail to get a majority if one in three pro-remain voters in England and Wales switched their vote.

In this scenario, the Conservatives would win 309 seats, Labour 233, the Liberal Democrats 34, Plaid Cymru four and the Greens one. When the Scottish National party, Democratic Unionist party and the Commons Speaker are factored in, this would give pro-remain parties a slim majority.

Smith said: “If we vote tactically we can stop a Boris Johnson majority and return a parliament that much more accurately reflects the state of the country’s views on the issue of Europe, which is now a majority pro-European country and we need a majority pro-European parliament.

“We think that there are only 20% of people who are so committed to the way they are going to vote that they cannot be persuaded to vote tactically. If you can get 30% of pro-remain voters to hold their nose and vote for a party they don’t normally vote for in their seat to stop the Conservatives or Brexit party, you get over that golden 320 figure to form a government. At 40%, we are way over.”

How do I register to vote in the 12 December 2019 general election? Read more

If 40% of pro-remain voters chose to vote tactically, it would put the Tories on 277 seats, Labour on 254 and the Lib Dems on 44, securing a large remain majority. In both scenarios the SNP would win 52 seats.

Best for Britain’s assessment was done on a seat-by-seat analysis with a poll of 46,000 British people in September and October. Without any tactical voting, the Tories would win a majority of 44, taking 364 seats to Labour’s 189, it said.

There are concerns that the remain vote is split across five political parties, including Labour, while the leave vote is split between the Tories and Brexit party.

Lewis Baston, an election expert, said at a launch event for the study in Westminster that in the current climate casting an “effective vote” was “all the more important because the remain tribe is more divided than the leave tribe”.

Votes are likely to be split so markedly between a range of parties that there will be a dramatic reduction in the number of MPs elected with more than 50% of the local vote. Baston predicted this would fall from 242 Conservative and 221 Labour MPs currently to just one Tory and 18 for Labour.

Baston said: “We know from all the social research and academic research that has been done [that] leave and remain identities are very strong and often override their party loyalties.”

Baston said the elections in 2015 and 2017 were hard to predict because of the number of people switching parties, and he expected the same to be true in 2019.

He said: “Between successive elections, extraordinary numbers of people changed their minds in the 2015 and 2017 elections. Something like 35 to 45% of people were switching over from one to another. We’ve never seen levels of volatility at an individual level since the 1920s. We can’t just look back at 2017 or 2015 or 2010 to look at the geography of party support.”