The ex-justice secretary became the campaign’s most high-profile ex-Tory supporter, urging voters to deliver a hung parliament to deny his former boss the chance to pass his “disastrous” deal.

“My view is the way forward now is to have a confirmatory referendum on Boris Johnson’s deal,” Mr Gauke said.

He argued the public looking at the deal on offer “now know what it is”, adding: “It is a harder Brexit than was promised to the British people in 2016.

“Because the consequences of the Boris Johnson deal are so significant, we do need to check back in with the British people – and I think it is perfectly possible for there to be a majority for that after the general election.”

Mr Gauke – the ultimate cabinet loyalist until just four months ago – announced he would run against the Conservative candidate in his ultra-safe constituency of South West Hertfordshire.

The ball is now in Jo Swinson’s court to decide whether the Liberal Democrats will give him a free run, as it will for two other ex-Tories, Dominic Grieve and Anna Soubry.

“It would obviously make my life easier if they were to do that, but that is a decision for them,” Mr Gauke told BBC Radio 4’s Today programme.

The Lib Dems had refused to stand aside for any candidate opposed to a second referendum, but could now agree to do so after Mr Gauke swung behind the policy and campaign for Remain.

Final Say march: Independent's best photos Show all 70 1 /70 Final Say march: Independent's best photos Final Say march: Independent's best photos People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Final Say march: Independent's best photos People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Final Say march: Independent's best photos People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Final Say march: Independent's best photos People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Final Say march: Independent's best photos People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Final Say march: Independent's best photos People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Final Say march: Independent's best photos People's Vote Final say march - 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The former minister is most alarmed by what he fears is the growing inevitability of a no-deal Brexit in just over a year, if Mr Johnson wins and passes his exit deal.

Nigel Farage pointed to that scenario when explaining his sudden decision to stand down Brexit Party candidates in all 317 Tory-held seats, in a significant boost to the Conservative campaign.

Hours earlier, Mr Johnson had issued a video insisting he would not extend the brief post-Brexit transition period – during which the UK will abide by EU rules – beyond December 2020.

It means the UK would lose all economic and security ties with the EU unless a new complex deal can be struck in time – a prospect dismissed by trade experts as all but impossible.

Mr Gauke said: “A Conservative majority after the next general election will take us in the direction of a very hard Brexit and in all likelihood, at end of 2020, we will leave the implementation period without a deal with the European Union on WTO [World Trade Organisation] terms – in effect, on no-deal terms.

“And that I believe would be disastrous for the prosperity of this country.”