The Prime Minister has confirmed he "absolutely" does have full confidence in Home Secretary Priti Patel following allegations she led a campaign to oust the Home Office's chief civil servant.
Sir Philip Rutnam, the department's most senior civil servant, quit on Saturday after accusing Ms Patel of orchestrating a "vicious" campaign against him, of lying about her involvement in it and of creating a climate of fear among her officials.
Sir Philip said he was quitting to "protect the health, safety and wellbeing" of the 35,000 staff in his department.
Speaking on Sunday, Boris Johnson said Ms Patel is a "fantastic Home Secretary.
"It is never an easy job, as anyone who has been Home Secretary will testify.
"It is one of the toughest jobs in Government."
He added he was "full of admiration" for the work of the civil service:
"We politicians could not begin to accomplish things without the fantastic, absolutely brilliant alpha minds in the civil service.
"We depend on them."
The PM's comments came as current and former ministers rallied around Ms Patel, although she has not spoken publicly since Sir Philip's resignation.
Health Secretary Matt Hancock said Ms Patel was "extremely courteous" in her dealings with colleagues, while Conservative ally Nusrat Ghani MP, who was sacked as a minister by Mr Johnson last month, said the criticism of Ms Patel had sexist overtones.
ITV News Political Correspondent Romilly Weeks says Priti Patel "faces multiple problems" including an employment tribunal and calls from MPs for the Home Secretary to face them in the Commons
Their comments come almost 24 hours after Sir Philip's sensational resignation, a period during which no minister spoke out in defence of the embattled former international development secretary.
Mr Hancock told the BBC's Andrew Marr programme: "Priti is a very determined Home Secretary.
"She is probably closer to where the public are on the issues of law and order than any home secretary in recent history.
"I think she drives things forward.
"I also think she is extremely courteous, and in every dealing I've had with her she has been very courteous.
"I can't get into the details of the case, not least because it looks like it might be going to court, and I'm not close to it."
Asked on BBC Radio 5 Live whether he was accusing Sir Philip of lying, Mr Hancock added: "I'm not getting into that."
Former transport minister Ms Ghani used her appearance on Sky News to suggest that Ms Patel could be being judged more harshly because she is a woman.
She said: "My experience of working with Priti is that she is incredibly determined.
"She provides strong leadership and she's delivering some very important aspects of our policy.
"I do think it's curious that if you're providing leadership, if you're determined, working at a fantastic pace, that within men that is seen as a fantastic skill and for women sometimes it's seen as challenging."
Labour has looked to ramp up the pressure on the under-fire Home Secretary, with shadow chancellor John McDonnell suggesting she could be forced to resign.
Mr McDonnell, when asked if Ms Patel could stay in her post, told Sky News' Sophy Ridge on Sunday: "I can't see it, it's bizarre."
He added: "One of the ways the Prime Minister could go forward is to have an independent investigation but during that period you'd have to suspend the Home Secretary whilst that went on."
Labour leadership candidate Sir Keir Starmer also called on Ms Patel to explain to MPs the explosive allegations levelled against her by Sir Philip.
Sir Keir said: "The Home Secretary has a duty to come to Parliament on Monday to explain the allegations made about her own conduct."
The shadow Brexit secretary called for Cabinet Secretary Sir Mark Sedwill to start "an immediate investigation" into the circumstances surrounding Sir Philip's departure.
In his statement on Saturday, Sir Philip said the campaign against him included "false" claims that he had briefed the media against the Home Secretary.
He said: "The Home Secretary categorically denied any involvement in this campaign to the Cabinet Office.
"I regret I do not believe her. She has not made the effort I would expect to dissociate herself from the comments.
"I believe these events give me very strong grounds to claim constructive, unfair dismissal and I will be pursuing that claim in the courts."
Cabinet Secretary Sir Mark issued a brief statement about two hours after Sir Philip quit, saying he had accepted his resignation "with great regret", and adding: "I thank him for his long and dedicated career of public service".
Ms Patel has not publicly spoken since Sir Philip's resignation but has previously denied mistreating staff.