Boris Johnson’s bounce is falling flat in key marginal seats with the Conservatives set to lose more than half the constituencies they need to defend against the resurgent Liberal Democrats.

A YouGov poll of more than 1,200 voters in 20 constituencies with small Tory majorities, where the Lib Dems had come second in 2017, shows a 14.1% slump in Tory support.

The poll, revealing a swing of more than 8% to the Lib Dems since the 2017 election, will act as a warning to Johnson, who has put his party on an election footing.

The swing increases yet further if he campaigns for a no-deal Brexit. If parties opposed to no deal were to unite on a single platform, the Tories would be defeated in