Boris Johnson has been warned he faces “constitutional crisis” over his “do or die” Brexit plans within weeks of entering Downing Street, ahead of his expected confirmation as Britain’s next prime minister on Tuesday.

His anticipated election by an estimated 160,000 Conservative Party members will be followed by a government walkout, with further pro-EU ministers expected to follow chancellor Philip Hammond, justice secretary David Gauke and international development secretary Rory Stewart in resigning before they can be sacked.

And Sir John Major became the third former PM in as many days to issue a stern warning about a Johnson premiership.

Whoever succeeds Theresa May as prime minister “must choose whether to be the spokesman for an ultra-Brexit faction or the servant of the nation he leads,” said the former Tory premier. “He cannot be both, and the choice he makes will define his premiership from the moment of its birth.”

Mr Johnson narrowly avoided being blocked at the last minute from fulfilling his lifetime ambition of becoming PM.

Boris Johnson's famous relatives Show all 11 1 /11 Boris Johnson's famous relatives Boris Johnson's famous relatives 1. King George II of Great Britain and Ireland, Elector of Hanover (1683 to 1760) Boris Johnson is a Hanoverian, and, thus distantly related to the Queen, David Cameron (via William IV) and Danny Dyer (via Edward III), among others. Boris's paternal grandmother, Yvonne Eileen Williams, known in the family as "Granny Butter" and whose family name was de Pfeffel, was a descendant of Prince Paul Von Wurttemberg. The German prince was, in turn, a direct descendant of George II. Discovered by genealogists f other BBC’s Who Do You Think You Are, Johnson commented, in 2008: "I felt I was the product of newcomers to Britain so it is totally bizarre, surreal in fact, to be told that in fact my Great x 8 Granddad is George II. But don't neglect the point that he shares that distinction with 1,023 others – there must be several thousand other people out there in the same position.” National Portrait Gallery Boris Johnson's famous relatives 2. The “Mummy of Basel”, Anna Catharina Bischoff (1719 to 1787) Last year, scientists in the Swiss city of Basel solved a decades-old mystery over the identity of a mummified woman. DNA extracted from the mummy’s gig toe indicates that the female is a great-great-great-great-great-great-great grandmother of Boris Johnson. The body was uncovered in 1975 while renovations were being done on Basel's Barfüsser Church, and was buried at the altar, wearing fine clothes, indicating she was at least well-to-do if not nobility. High levels of mercury in her remains suggested she had been treated for syphilis (the metal also helped preserve her). National Gallery of Basel Boris Johnson's famous relatives 3. Ali Kemal (1867 to 1922) (Pictured with wife Winifred Brun) For a man who made so much capital in the 2016 referendum on the prospect of Turkey joining the EU and its 80 million citizens thus enjoying free movement to the UK, Boris Johnson sometimes makes a surprisingly big deal of his Turkish Muslim great-grandfather on his father’s side, who he claims was an asylum seeker. Ali Kemal, according to his famous descendant, came to Britain because it was “a beacon of generosity and openness”. I t might be overstating it, but he did live in exile in England for a time. Unknown Boris Johnson's famous relatives 4. George Williams (1821 to 1905) Sir George, as he became, is the great (x4) grandfather of Boris Johnson, and was one of the founders of the Young Men’s Christian Association or YMCA, in 1841. An evangelical apostle of “muscular Christianity”, George took it upon himself to organise some fellow drapers and establish a safe place for young men where they could be shielded from the debauchery and the temptations of the flesh and the grape. No sofas would suffer red wine stains in the hostel. Since then it has gone global, today assisting 58 million people across 119 countries, which is almost as many as Boris helps. A social visionary of his time, George was knighted for his works by Queen Victoria in 1894. National Portrait Gallery Boris Johnson's famous relatives 6. King Friedrich of Wurttemberg (1754 to 1816) Though stocky of build, and handy in a game of rugger, Boris Johnson is not especially heavy or tall. This ancestor was. King Friedrich stood 6 foot 11 inches, and weighed 31 stone (2.12 metres/200 kilograms). Napoleon remarked that God had created the Prince to demonstrate the utmost extent to which the human skin could be stretched without bursting. There are rumours that he was bisexual and enjoyed the close companionship of young noblemen. This added to the strains on his marriage to Augusta, who was the granddaughter of King George II. One of their four children, Prince Paul is the link to the Johnsons, via an illegitimate daughter he fathered in Paris with an actor named Friederike Margrethe Porth. Ludwigsburg Castle Archive Boris Johnson's famous relatives 7. Professor Elias Lowe (1879 to 1969) Elias is Boris Johnson’s mother Charlotte’s great grandfather. The distinguished Princeton scholar and student of ancient scriptures (palaeographer) , Elias arrived in the United States as a refugee from Lithuania in 1891, and was affine of Albert Einstein. Jewish, Lowe came for a line of revered rabbis. Although he cannot be counted Hallachially Jewish, the Jewish Chronicle makes him 5 per cent Jewish on their reckoning. Rutgers School of Arts and Sciences Boris Johnson's famous relatives 8. Helen Lowe-Porter (1876 to 1963) Helen Tracey Lowe-Porter. is Boris Johnson's mother Charlotte’s great grandmother. An American, she married the Lithuanian-born academic Elisa Lowe, and is said to have been probably the most prominent literary translator in the English-speaking world working from German to English in the twentieth century. However, not necessarily the best and in such circles her reputation is contested. In any event, she retained for 50 years the exclusive rights to translate the works of her friend Thomas Mann. Her and Elias’ daughter Beatrice is Charlotte Johnson (nee Fawcett’s) mother. Lowe-Porter family Boris Johnson's famous relatives 9. Sir Henry Fawcett MP (1833 to 1884) Before Boris and Jo Johnson became MPs and minsters, there was Sir Henry Fawcett – Britain’s first blind MP. He was the husband of the famous suffragette Millicent Garrett Fawcett, and thus an ancestor of Boris on his mother’s side - though the family tree isn’t clear on how close they are related. Glasgow University Boris Johnson's famous relatives 10. Prince Paul of Wurttemberg (1785 to 1852) Odd looking, an amusing womaniser (remind you of anyone?), this minor German aristocrat was the progenitor of the Johnson’s posh pedigree, such as it is. His affair with an actress is Paris, Fredericke Porth, gave rise to a daughter (out of wedlock as they used to say) provided the link back to the royal families of Wurttemberg and Hanover, and thus of Great Britain. By the same token it means that Stanley, Boris, Rachel, Leo and Jo, and the rest of them along that branch of the tree, are also distantly related to most of the royal families of Europe including the Russian Romanovs – Johnson stands connected, albeit tenuously, to the Belgian, Danish, Dutch, Luxembourg, Norwegian and Swedish families, plus the German Kaiser. Paul had five declared children, and two illegitimate ones, at least that are known about. National Archive Holland Boris Johnson's famous relatives 11. Fredericke Porth (1777 to 1860) When, on the BBC show Who Do You Think You Are? Boris Johnson discovered the identity of his 4x Great Grandmother, Fredericke, he was just a touch chauvinist: “An actress, could be a euphemism we may be about to turn up a prostitute here. Not that I mind. I want you to know they can get up to anything, my ancestors, they have carte blanche to commit whatever acts of fornication they want as far as I am concerned, but I want to know”. It seems Fredericke Margarethe was indeed an actress for most of her life, and was widowed by the time her illegitimate daughter, the product of her affair with Prince Paul of Wurttemberg was born, in 1805. Born Porth, Fredericke was married to a man named Vohs until 1804, and, in 1818, remarried to a man named Werdy. She was described as a “Royal Saxon Court-Actress”. Alamy Stock Photo Boris Johnson's famous relatives 12. Millicent Garrett Fawcett (1847 to 1929) Disappointingly, the ancestor who is sometimes mentioned as a stands as a standing genealogical reproach to Boris Johnson may not be a related at all. As a pioneering feminist and suffragette, she’d surely disapprove of Boris’ attitudes towards womankind. As President of the National Union of Women’s Suffrage Societies (NUWSS), the largest component of the suffragette movement, she did as much as anyone to get women into the political life of the nation, and the Fawcett Society, still fighting for equal human rights, is named in her honour. Millicent lived just long enough to see the vote being granted on an equal basis to all women, and said this when it was finally enacted in 1928: “It is almost exactly 61 years ago since I heard John Stuart Mill introduce his suffrage amendment to the Reform Bill on May 20th, 1867. So I have had extraordinary good luck in having seen the struggle from the beginning.” Bain News Service/Elliott & Fry

His former deputy at the Foreign Office, Sir Alan Duncan, quit the government in order to table a motion in the Commons to test whether the new Tory leader could command a majority of MPs.

The motion was rejected by Speaker John Bercow, but if it had been allowed to go to a vote on Wednesday, it might have prevented Ms May from recommending to the Queen that Johnson was the person best able to form a government after her departure.

Unless his rival Jeremy Hunt pulls off one of the most remarkable political turnarounds of modern times, Mr Johnson is expected to be confirmed as new Tory leader around noon on Tuesday.

But Ms May will remain in office for one more day and face her final session of Prime Minister’s Questions in the Commons on Wednesday before going to Buckingham Palace to offer her resignation to the Queen. Moments later, her successor – almost certainly Mr Johnson – will follow her into the palace to “kiss hands” with the monarch.

Alan Duncan explains his emergency motion to try to stop Boris Johnson becoming PM

The former foreign secretary will move quickly to appoint a cabinet to take forward his promise to take the UK out of the EU by the end of October, deal or no deal, in a parliament where he commands a working majority of just four – expected to be reduced to three by next week’s by-election in Brecon and Radnorshire.

He is likely to issue an appeal for unity in a Conservative Party which has become mired in vicious infighting over Europe under Ms May’s leadership.

Former ministers warned that he risks plunging the party into further turmoil unless he reflects the range of opinion on Brexit in his top team.

Guto Bebb, the former defence minister, quit the government over Brexit (PA)

“He has been surrounded by members of the parliamentary party who have extreme views about the appropriateness of a no-deal Brexit,” said Guto Bebb, who quit the government over Brexit last year. “I’m not convinced he can pivot back to the centre, and that is going to bring the possibility of a no-deal outcome closer.”

The Aberconwy MP told The Independent it was a “fallacy” to believe there was a majority to be won in the Commons or the country for no-deal Brexit. But he said last week’s victory for a motion preventing Johnson from suspending parliament to force no deal through meant he was unlikely to face a no-confidence vote before the autumn.

“People are taking the view that he can be given the summer to travel around, giving the impression of activity while not necessarily achieving very much,” he said. “But I think we will be in a pretty severe constitutional crisis within six to eight weeks.”

Former culture minister Margot James, who walked out of the government last week over Mr Johnson’s threat to suspend parliament, said she expected Tory MPs to give Mr Johnson an opportunity to show he can negotiate the acceptable withdrawal deal that eluded Ms May.

“A lot of us will do all we can to support him in a genuine attempt to get a deal that will pass through parliament and be acceptable to the EU and Ireland,” she said.

“If he can do that, he will have my support. But I’m certainly not prepared to catapult out of the EU without a deal, which would be completely contrary to some of our manifesto commitments and a lot of what was said in the referendum as well.”

Steve Brine, who quit as a health minister in March to vote against the government on Brexit, said Johnson’s future may depend on whether he governs as “good Boris” or “bad Boris”.

“He will start with a clean slate as far as I’m concerned,” said Hunt supporter Mr Brine, who stressed he had not given up hope of victory for the foreign secretary. “If he appears to be a unifier and reaches across different sections of the parliamentary party, then I think people will be willing to give him a chance. A lot depends on who he appoints to senior cabinet positions – that will show how conciliatory he intends to be.”

Jacob Rees-Mogg, a prominent Brexiteer, said Mr Johnson was the right person to make the Tories “electable” and said he would be ready to serve in any position in his administration.

“I think he’s the right answer, I think he’ll be a very good prime minister,” the chair of the influential European Research Group told LBC. “Charisma is the stardust of politics. It’s difficult to define. It’s difficult to pin down. Boris just has it and other people – almost everyone else – don’t have it.

“I think he will have the right spirit in getting us out of the European Union and moving onto the other things that we need to do. So if he wants me to be the junior under-minister in Pyongyang, I will happily go off and do it.”

Sir Alan, who served for two years under Mr Johnson at the Foreign Office, made clear he would not serve under a man he has described as a “circus act” and said it was tragic that the UK’s influence in the world was being undermined by “the dark cloud of Brexit”.

He insisted his actions were not motivated by “personal animosity of any sort” towards Mr Johnson but told the BBC: “I have very grave concerns that he flies by the seat of his pants and it’s all a bit haphazard and ramshackle.”