SIR John Major has become the third former prime minister in 24 hours to issue a Brexit warning to Boris Johnson, cautioning him about the dangers of becoming the spokesman for an “ultra-Brexit faction,” should he take over in No 10 on Wednesday.

The former Conservative premier was responding to a speech by Gordon Brown, who accused Mr Johnson of preparing to push the British economy "off a cliff" with his determination to deliver Brexit by the end of October "do or die".

Sir John said he had read the former Labour leader’s speech with interest and agreement and stressed that whoever succeeded Theresa May this week would be far more than just the leader of the Conservative Party.

“As for every prime minister, he must act for our nation as a whole; not just one part of it. He must also remember that no-one born this century voted for Brexit let alone a no-deal Brexit.

“Words and actions have consequences and never more so than when they are those of the Prime Minister. As the evidence mounts of the probable economic and social damage of a no-deal Brexit – and of the rising opposition to it – the new Prime Minister must choose whether to be the spokesman for an ultra-Brexit faction or the servant of the nation he leads.

“He cannot be both and the choice he makes will define his premiership from the moment of its birth,” declared Sir John.

He said that as the most powerful politician in the four nations of our UK, any prime minister had the right to expect support but – if he acted as the spokesman for only one hard-line faction – he could not complain if he faced uncompromising opposition from those who believed they had had their views ignored.

“I hope our new leader understands this and is fully prepared for the enormity of the task before him,” added Sir John.

Earlier, Mr Brown warned that a no-deal break from the EU would be a "self-inflicted wound," comparable in military terms to the Charge of the Light Brigade.

In a speech to the Hope Not Hate campaign in London, the Scot warned that the Brexiteers were in danger of descending into a brand of "paranoid nationalism" which risked breaking up the UK.

He said Mr Johnson might 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," declared the former PM.

"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 per cent rise is the latest estimate - and by Monday, the pound - already sharply down on its pre-Brexit value - will be under pressure.

"By Tuesday, medical drugs from mainland Europe will be less accessible and a week after Brexit companies will be complaining that vital stocks and components are not reaching them, and that is likely to put their workers on short-time."

Mr Brown said that polling by Hope Not Hate suggested that three million voters who backed Leave in the 2106 referendum now opposed a no-deal Brexit.

However, he said that the Brexiteers, driven by "blind faith, dogma and emotion" showed little consideration for the consequences of what they were proposing.

"When future historians look back, they will be shocked to discover how such an act of economic self-harm that runs wholly counter to the national interest could ever be portrayed by Nigel Farage and Boris Johnson as the height of patriotism, and criticism from any quarter be dismissed as a betrayal of Britain and all we stand for," he said.

"British history includes self-inflicted wounds - military disasters such as the Charge of the Light Brigade and the fiasco of Gallipoli - but no peacetime act of self-harm can rival a no-deal Brexit for which we are so woefully unprepared.

"Brexiteers may be trying to reinvent a 'Britain alone' Dunkirk spirit, the Britain of indomitable fortitude, but all too easily this hijacking of patriotism descends into an inward-looking, intolerant and adversarial brand of paranoid nationalism hell-bent on blaming all who disagree," added Mr Brown.

Meanwhile, 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," wrote the former PM in an article for The Times.

"If he doesn't back down from his stated negotiating position, he will fail," declared the former Labour leader.

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

Mr Blair went on: "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 argued 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 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," he insisted.

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