Conservative Party MP Nusrat Ghani had said in a ‘Sunday Times’ interview that at a post-reshuffle meeting in February 2020 with the party whips, or MPs in charge of party discipline, she was told that her “‘Muslimness’ was raised as an ‘issue’, that my ‘Muslim woman minister’ status was making colleagues uncomfortable”.

Chief Tory Whip Mark Spencer said over the weekend that he was the whip in question and described Ghani’s allegations as "completely false".

On Monday, Downing Street confirmed that Johnson has asked the Cabinet Office to carry out an inquiry into the allegations.

"At the time these allegations were first made, the prime minister recommended to her that she make a formal complaint to CCHQ (Conservative Campaign Headquarters). She did not take up this offer," a Downing Street spokesperson said.

"The Prime Minister has now asked officials to establish the facts about what happened. As he said at the time, the Prime Minister takes these claims very seriously," the spokesperson said.

Ghani welcomed the investigation in a statement on Twitter, indicating that she had spoken to Johnson on Sunday night.

She said: “As I said to the Prime Minister last night all I want is for this to be taken seriously and for him to investigate. I welcome his decision to do that now.

"The terms of reference of the inquiry must include all that was said in Downing Street and by the whip. I look forward to seeing the terms of reference."

The new inquiry comes at the start of a crucial week for Johnson’s leadership. A separate Cabinet Office inquiry, led by top civil servant Sue Gray into parties and gatherings in Downing Street and other government departments during COVID-19 restrictions in 2020 and 2021, is expected to conclude this week with a report.

If Johnson is found to have legally breached lockdown rules in the so-called “partygate” scandal, the calls from within his own party and the Opposition for him to step down as prime minister will intensify once again.

According to reports so far, 16 gatherings allegedly took place in either No. 10 Downing Street – Johnson’s home and residence – or other government departments during the height of the COVID pandemic.

Among the central events being investigated by Gray is a Downing Street garden “bring your own booze” event on May 20, 2020, when legal restrictions and guidelines were in place to limit the spread of coronavirus.

Johnson has apologised to the House of Commons for attending this gathering but stressed that he believed it was a "work event" where he stayed for 25 minutes to thank staff for their efforts.

The UK PM has told Parliament that he will return to the Commons to make a statement at the end of the internal inquiry process.