An early general election could result in Boris Johnson losing more than half of 20 key marginals in Conservative and Liberal Democrat battlegrounds, according to a new analysis.

Highlighting the risk of heading to the polls at a snap election for the prime minister, the survey shows the unequivocally anti-Brexit party could swipe seats from the Tories, predominately in the south of England.

It comes amid heightened speculation of an early election in the autumn, either called by Mr Johnson himself or forced upon him by MPs voting to topple his new administration in a vote of no confidence after the summer recess.

In a letter to the entire civil service on Friday, Mr Johnson said it was now No 10’s “top priority” to prepare the country for leaving the EU without a deal on 31 October, adding he “recognised” a new Brexit deal may not be achieved.

The polling by YouGov for the People’s Vote campaign – covering 20 constituencies with the smallest Tory majorities where the Lib Dems came second in 2017 – shows 11 of them of could fall from Tory control.

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It suggests 30 per cent of those who backed Theresa May at the 2017 snap election would no longer do so. Of these, 10 per cent said they would now back the Lib Dems while a further 18 per cent would vote for the Brexit Party under Nigel Farage’s stewardship.

One of the constituencies included in the data – Brecon and Radnorshire – fell to the Liberal Democrats from Mr Johnson’s party just two weeks’ ago in a by-election that reduced the government’s working majority in the Commons to just one.

The People’s Vote campaign said Tories at risk include the Richmond Park MP Zac Goldsmith, who regained his seat from the Liberal Democrats at the election called by Theresa May in 2017.

Others constituencies such as St Ives, Cheltenham, North Devon, Cheadle, Lewes, St Albans and Hazel Grove could also fall to Jo Swinson’s party.

The pollsters also asked voters how they would cast their ballot at an early general election should the Tories insist on a no-deal Brexit and the other major political parties offered a new referendum and opposed a no deal. In this scenario, 13 of the 20 key marginals would turn yellow.

Respondents in these regions also back a Final Say referendum by 56 per cent when “don’t knows” are excluded, with 44 per cent opposed.

Peter Kellner, the former president of polling company YouGov, said the survey showed that in the Lib Dem-Tory battleground seats, the prime minister’s “hard line on Brexit is far from a deal sealer in any early election”.

“Even before a campaign has begun, when the Lib Dems could expect to benefit from enhanced coverage and their tactical voting message will almost certainly gain traction, Jo Swinson’s party is well-placed to make significant gains at the Tories’ expense.”

Mr Kellner continued: “Given the volatility of recent general election campaigns it’s hard to see Boris Johnson opting for an early election as anything other than a mad gamble.

“Of course, he’s always shown himself willing to take risks, but a lot of Conservative MPs could face a very premature end of their political career even if nothing changes from this poll.

“In fact, there are very good reasons to believe a poll like this underestimates the real level of Lib Dem support because that party has generally shown itself to be highly effective in its target seats at squeezing Labour and Green votes to block the Tories.”