Former prime ministers Tony Blair and Gordon Brown have issued a warning to the next occupant of Number 10 about the dangers of a no-deal Brexit.

Boris Johnson, who is widely expected to triumph in the Tory leadership contest and become prime minister on Wednesday, has promised to leave the European Union on October 31 come what may, even if that means a no-deal exit.

In separate interventions, the Labour ex-premiers warned that break from Brussels without a deal could be "catastrophic" or see the British economy "pushed off a cliff".

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Writing in The Times, Mr Blair said another referendum was the best outcome for Mr Johnson.

"Even before he becomes prime minister this week, Boris Johnson has boxed himself in to a no-deal Brexit," Mr Blair said.

"If he doesn't back down from his stated negotiating position, he will fail."

He said the 2016 referendum gave the Government a mandate for Brexit, but not a no-deal scenario, Mr Blair claimed, and "crashing out without public endorsement would be a gargantuan risk".

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"No-one knows with certainty the impact of no-deal for the simple reason that no developed nation has ever left overnight its preferential trading arrangements in this manner.

"It could be merely very difficult or it could be catastrophic."

If Parliament voted against a no-deal Brexit, Mr Blair said he did not believe Mr Johnson would push on regardless - meaning a referendum or a general election would be required.

Mr Blair said he could understand why Mr Johnson might push for a general election against Jeremy Corbyn - there is no love lost between the former prime minister and the left-wing Labour leader - but that "ploy" would be "completely unjustifiable" because it would be about issues other than just Brexit.

"Brexit is an issue that stands on its own. It should be decided that way," Mr Blair said.

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Neither Mr Johnson nor his leadership rival, Jeremy Hunt, have ruled out a no-deal Brexit if agreement cannot be reached with Brussels.

In his own warning, Mr Brown highlighted that polling suggesting support for a no-deal Brexit was declining.

Mr Brown said: "Boris Johnson is becoming prime minister just as support for his October 31 no-deal policy is falling away.

"Already, away from Westminster, three million Brexit voters will not support a no-deal Brexit.

"Many more are ready to say 'no' to a no-deal and to seeing the British economy pushed off a cliff."

He said that Mr Johnson may be fated to be remembered by history “not as the 55th prime minister of the UK but as the first prime minister of England”.

Despite being chosen by an electorate smaller than that which voted for Ed Balls on Strictly Come Dancing, Mr Brown said the Tory leadership favourite was heading for a “chaotic, damaging and dangerous” no-deal break.

“The message to Boris Johnson is plain and urgent: Don’t push Britain off a cliff on October 31,” he said.

“If no-deal goes ahead on Thursday October 31, 24 hours later – on what Brexiteers will call ‘freedom Friday’, but others ‘black Friday’ – there will almost certainly be hold-ups at Dover; by Saturday, pile-ups on our motorways; by Sunday, food prices will be going up – a 10% rise is the latest estimate – and by Monday, the pound – already sharply down on its pre-Brexit value – will be under pressure.