The former foreign secretary, who has confirmed he will stand in the Tory leadership race, has long eyed the job despite doubts among his party colleagues about his suitability.
If he moves into Downing Street, he will carry with him the baggage of a lengthy history of racially inflammatory remarks that could complicate international diplomacy.
Muslim women ‘letter boxes’ jibe
In an August 2018 column for the Daily Telegraph, Johnson wrote that women who wore the niqab looked like ”letter boxes” and “bank robbers”. He suggested it was “absolutely ridiculous” that people would choose to wear the “oppressive” religious headgear.
His comments infuriated other Tories and led Labour MP David Lammy to accuse him of “fanning the flames of Islamophobia to propel his grubby electoral ambitions”.
But the Conservative Party cleared him of breaching its code of conduct after an independent panel ruled his column was “respectful and tolerant”.
Picanninnies and ‘watermelon smiles’
In another Telegraph column, this one written in 2002, Johnson used two racial slurs to refer African people.
Tony Blair would love touring the continent, he wrote, because he will be greeted by “cheering crowds of flag-waving piccaninnies” – a racist term for a dark-skinned African child - and “the tribal warriors will all break out in watermelon smiles to see the big white chief”.
He apologised six years later during a London mayoral contest, though he claimed his words had been taken out of context.
After Barack Obama intervened in Britain’s EU debate, Johnson suggested the then US president had an “ancestral dislike of the British empire” because he was “part-Kenyan”.
The remark led to him being accused of “dog whistle racism”.
Papua New Guinea 'cannibalism'
Johnson was again forced to apologise after he suggested the people of Papua New Guinea were cannibals.
Discussing party politics, he said in 2006: “For 10 years we in the Tory Party have become used to Papua New Guinea-style orgies of cannibalism and chief-killing, and so it is with a happy amazement that we watch as the madness engulfs the Labour Party.”
The Pacific nation's high commissioner in London did not see the funny side, and demanded an apology.
Colonialism 'the best fate for Africa'
Writing in The Spectator in 2012, Mr Johnson suggested that Africans would be better off if ruled by white colonisers.
He said: "Consider Uganda, pearl of Africa, as an example of the British record. The British planted coffee and cotton and tobacco, and they were broadly right. If left to their own devices, the natives would rely on nothing but the instant carbohydrate gratification of the plantain.
“The best fate for Africa would be if the old colonial powers, or their citizens, scrambled once again in her direction; on the understanding that this time they will not be asked to feel guilty.”