The government is to investigate whether Home Secretary Priti Patel has breached the ministerial code, amid allegations of bullying.
Cabinet office minister Michael Gove confirmed the inquiry after an urgent question from Jeremy Corbyn.
It comes after bullying claims were made by the ex-top civil servant in Ms Patel’s department.
Mr Corbyn said he believed Ms Patel – who has previously denied she mistreated staff – should be sacked.
Sir Philip Rutnam, the Home Office’s most senior official, resigned on Saturday citing a “vicious and orchestrated” campaign against him.
Jeremy Corbyn: Government ‘led by bullies’
Jeremy Corbyn said if the “serious allegations” raised by Rutnam about the home secretary’s conduct were true “then that would clearly constitute a breach of the ministerial code”.
The Labour leader added: “Why, without a proper investigation, has the prime minister defended the home secretary, calling her fantastic and saying he absolutely has confidence in her?
“It’s not enough just to refer this to the Cabinet Office. The government must now call in an external lawyer as quite rightly suggested by the union for senior civil servants, the First Division Association.
“A minister in breach of the ministerial code cannot remain in office and should be dismissed.”
Corbyn said: “Isn’t the truth that this is a government led by bullies, presided over by a part-time prime minister who not only can’t be bothered to turn up but simply won’t take the vital action required when the very integrity and credibility of the government is on the line.”
Rutnam plans to take his case to an employment tribunal, which could call Patel, Johnson and Dominic Cummings, the prime minister’s chief aide, as witnesses.
Addressing the Commons on Monday, Gove said: “Allegations have been made that the home secretary has breached the ministerial code. The home secretary absolutely rejects these allegations.
“The prime minister has expressed his full confidence in her and having worked closely with the Home Secretary over a number of years, I have the highest regard for her – she is a superb minister doing a great job.
“This government always takes any complaints relating to the ministerial code seriously, and in line with the process set out in the ministerial code the prime minister has asked the cabinet office to establish the facts.
“As is usual, the independent adviser on ministerial interests, Sir Alex Allan, is available to provide advice to the Prime Minister.”
Gove said the government would not comment on an individual matter, in line with its long-standing policy, before praising the Home Office ministers and civil servants.
Formal complaint about Ms Patel’s conduct was made when she was employment minister.
The BBC’s home affairs correspondent Danny Shaw has also learnt that a formal complaint about Ms Patel’s conduct was made when she was employment minister at the Department for Work and Pensions. The substance of it is not known, nor whether it was substantiated or followed up.
The complaint is believed to have been made by a member of her private office – a team of six to eight civil servants which works closely with an individual minister.
A spokesman for Ms Patel said she was “not aware” of the complaint and the government, while it did not deny the claim, said it would not comment on personnel issues.
One Whitehall insider said Ms Patel had created a “hostile and unhappy” environment for civil servants there by questioning their capability and undermining their performance.
“I felt very sorry for people in her private office – they felt bullied,” they said.
BBC political correspondent Iain Watson said allies of Ms Patel are privately suggesting that Sir Philip was not up to the demands of the job.