Boris Johnson will be denied a majority if just 30% of pro-Remain voters act tactically at the general election, an anti-Brexit campaign group has calculated.
According to Best for Britain, the prime minister is on course to win 364 seats – a comfortable majority of 44 – if voters do not vote tactically.
But if 30% abandon party loyalty to unite behind a pro-Remain candidate, the Tories would win just 309 seats.
The analysis published on Wednesday suggested in this scenario Labour would win 233 seats, the Lib Dems would win 34 seats, Plaid Cymru would win four and the Green Party would win one.
The result would give parliament a pro-Remain majority of four.
Best for Britain conducted a huge seat-by-seat analysis of 46,000 people over September and October.
The data assumes the SNP will win 52 seats in Scotland.
If 40% of voters decide to tactically back candidates who support a second referendum, the survey predicts parliament would have a pro-Remain majority of 36.
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Dominic Grieve, the former Tory MP who was stripped of the party whip after voting to block a no-deal Brexit, today promoted a new tool targeted at pro-Remain voters to help them choose who to vote for.
The former attorney general, who intends to stand as an independent in his Beaconsfield constituency in the upcoming election, acknowledged he would be a “direct beneficiary” of pro-EU tactical voting.
“Brexit must be the defining issue of this election,” he said. “If Boris Johnson gets a working majority then he will doubtless take us out of the EU very quickly.”
Grieve confirmed the Lib Dems had agreed to stand aside in his seat in order to help him get elected to stop the “terrifyingly bad deal”.
And he said while Labour was likely to stand a candidate, he had been told “a number of members” of the local Labour Party would “choose to vote for me”.
It comes as political parties are gearing up for the first December general election in almost a century.
Johnson has said he is prepared for a “tough” battle, while Shadow Chancellor John McDonnell said he thought Labour would win a majority.
Lib Dem leader Jo Swinson insisted the campaign was not a two-horse race between Johnson and Jeremy Corbyn, and that she was standing as a candidate to be prime minister.
The government’s one-page bill enabling the election to be held on December 12 will be considered by the House of Lords on Wednesday, but it is unlikely to be held up in the unelected upper chamber.