The Pakistani-origin politician Sajid Javid has already made his mark by becoming Britain’s first Asian origin home secretary, regarded as one of “the great offices of state”, along with Prime Minister, foreign secretary and chancellor of the exchequer.

But the 49-year-old former banker, who is a millionaire, wants to go further and make history as the country’s first Prime Minister from an ethnic minority background — he is currently one of 12 candidates, including Boris Johnson, who have declared they are standing to replace Theresa May.

She formally steps down on June 7 but will stay on as caretaker Prime Minister while the Conservative party chooses her successor by the end of July.

Javid is said to have a creditable “back story” — his father arrived from Pakistan and became a bus driver as was also the case with Sadiq Khan, the mayor of London. This caused former Prime Minister David Cameron to joke that “the sons of bus drivers are the new Etonians”. Javid has reckoned that if Sadiq, formerly the Labour MP for Tooting in South London, can win a mayor election, there is no reason why he cannot become Prime Minister. And he does have the support of some influential Indians.

But on Friday, Javid, who stripped Shamima Begum of her British nationality, was accused of taking this populist action to further his political ambitions.

Shamima, now 19, is a British Bangladeshi who ran away at the age of 15 with two of her classmates from her school in East London and joined the Islamic State terrorist group in Syria. She immediately “married” an IS fighter of Dutch origin and had three children, all of whom have died.

Legally, the home secretary cannot render a British national “stateless” which appears to be the case with Shamima because the Bangladeshi government has warned she is likely to be hanged as a terrorist if she set foot in Bangladesh.

This is not the first time Javid has been accused of using Shamima as political ammunition but on Friday, the contents of a hard-hitting 16-page letter written to the home secretary by the lawyer acting on behalf of her family were made public.

In a letter Mohammed Akunjee told the home secretary: “Your act represents the most profoundly egregious, capricious and politically-driven abuse of power.”

“It was a unilateral, unprincipled response to the publishing of (Times reporter) Mr (Anthony) Loyd’s interview (with Ms Begum in a refugee camp), deployed as an artifice or device to further your own personal political objective of being Prime Minister. Begum was a pawn to your vanity. Her baby died.”

The letter added: “Shamima Begum’s parents never contemplated a life for her in Bangladesh. They did not register her birth with the Bangladeshi High Commission.

“They did not take her to Bangladesh on holiday as a child. Indeed she has never visited the country. Rather, Shamima was born, raised, groomed and radicalised here in the UK.

“The suggestion that Shamima is to you genuinely a Bangladeshi citizen is unsustainable.

“It is the responsibility of a British Secretary of State to deal with British problems. Rather than take responsibility for Shamima Begum and her son, you took a British problem and illegally dumped it on our innocent international neighbours.

“You have dishonourably left it to the Kurdish people to bear the financial and security burden of Shamima’s safety and upkeep. You left them to minister to her sick child and to bury him. Through sleight of hand, you have sought to burden the Bangladeshis with her in the longer term. Your cynical decision amounts to human fly-tipping.”

The Home Office, who faces a court battle over the Shamima case, said it did not routinely comment in individual cases but the letter will make uncomfortable reading for Javid. It will also be used by his rivals to undermine his leadership credentials.