General election: Tactical voting in just 57 marginal seats could keep Boris Johnson out of Downing St, major poll indicates

Fewer than 120,000 anti-Brexit tactical votes in the right seats could deny Boris Johnson an overall majority in the House of Commons, new polling suggests.

A large-scale survey of almost 40,000 voters found that Conservatives are heading for 366 seats in the House of Commons, giving Mr Johnson a comfortable majority of 82.

But analysis for the Best for Britain campaign for a second EU referendum found that in 57 seats, the Tory candidate could be defeated by 4,000 or fewer anti-Brexit voters voting tactically.

And the campaign said that as few as 117,314 pro-EU tactical votes in the right seats could produce a hung parliament which could deliver a Final Say referendum.

The seat-by-seat analysis of 39,476 British adults was carried out between 15 October and 24 November by Focaldata, using the MRP technique to allow constituency-level forecasts.

It is the first MRP poll to adjust for the impact of Nigel Farage’s Brexit Party standing candidates down in 317 Conservative seats and 40 held by other parties, as well as the establishment of the Unite to Remain alliance between pro-referendum parties.

It found that roughly three-quarters of Brexit Party voters in seats where candidates were stood down have switched their support to the Tories.

Best for Britain’s analysis found that without tactical voting, Labour, Liberal Democrats, SNP, Greens and Plaid Cymru would hold a combined 265 seats in the 650-seat House of Commons.

But it also indicated that Tory seats are becoming increasingly vulnerable, with the number with a predicted margin of victory smaller than 5,000 votes rising from 131 in October to 165 now.

In 27 of these seats it would take less than 2,000 tactical votes to prevent a Conservative victory.

And in 14 Conservative-held seats, the projected margin of victory is less than 2 per cent.

In Keighley, it would take just 29 Green and Lib Dem voters to switch to Labour to defeat the Tory candidate.

Best for Britain said that if pro-EU voters in the key seats follow its tactical voting advice, the Conservatives would be reduced to 309 seats – a dozen short of a working majority. Labour would win 244 seats, SNP 52, Lib Dems 21, Plaid Cymru four and the Greens one.

Best for Britain chief executive Naomi Smith said: “Our data shows that tactical voting will be decisive at the upcoming election.

“Even with the Brexit Party collapse, there are still lots of seats in play for Remainers.

“This is crucial as it means they could be won by pro-EU parties if voters hold their nose and vote for the party with the best shot of beating the Tories.

“Having updated our recommendations just as the first wave of postal votes land on doorsteps, we’re confident that this election is up for grabs. We can stop Boris Johnson, and stop Brexit.”

Best for Britain is recommending a vote for Labour in 457 seats, Lib Dems in 104, SNP in 12, Green in three and Plaid Cymru in four.

The 57 key seats which they believe could prove decisive - with recommendations of which party anti-Brexit tactical voters should back - are:

Keighley (Lab)

Blyth Valley (Lab)

Wolverhampton North East (Lab)

Birmingham Edgbaston (Lab)

Enfield Southgate (Lab)

Warrington South (Lab)

Dewsbury (Lab)

Stoke-on-Trent Central (Lab)

Great Grimsby (Lab)

Walsall South (Lab)

Ynys Mon (Lab)

Bassetlaw (Lab)

Hyndburn (Lab)

Workington (Lab)

Bedford and Kempston (Lab)

Bolton North East (Lab)

Wolverhampton South West (Lab)

Blackpool South (Lab)

West Bromwich East (Lab)

Cardiff North (Lab)

Gordon (SNP)

Pudsey (Lab)

Banff and Buchan (SNP)

Crewe and Nantwich (Lab)

East Renfrewshire (SNP)

Burnley (Lab)

Alyn and Deeside (Lab)

Vale of Glamorgan (Lab)

Delyn (Lab)

Dagenham and Rainham (Lab)

Wirral West (Lab)

West Bromwich West (Lab)

Don Valley (Lab)

Scunthorpe (Lab)

Rother Valley (Lab)

Wrexham (Lab)

Bishop Auckland (Lab)

Darlington (Lab)

Vale of Clwyd (Lab)

Gower (Lab)

Moray (SNP)

Newcastle-under-Lyme (Lab)

Southport (Lab)

Ayr, Carrick and Cumnock (SNP)

Aberdeenshire West and Kincardine (SNP)

Calder Valley (Lab)

Putney (Lab)

Barrow and Furness (Lab)

Dumfries and Galloway (SNP)

Sedgefield (Lab)

Lincoln (Lab)

Lewes (Lib Dem)

Copeland (Lab)

Bolsover (Lab)

Hastings and Rye (Lab)

Aberconwy (Lab)

Bristol North West (Lab)