Boris Johnson has told MPs he is "sticking by" Home Secretary Priti Patel, following further allegations of bullying against her.
Claims she mistreated staff would "of course" be investigated, he said at Prime Minister's Questions.
But he hailed Ms Patel as an "outstanding" home secretary who was "delivering change".
The Cabinet Office is investigating several allegations about Ms Patel's behaviour, which she denies.
The home secretary, who sat next to Mr Johnson at PMQs, has not publicly commented on the allegations.
Answering a question from Labour MP Matthew Pennycook, Mr Johnson said: "The home secretary is doing an outstanding job delivering change, putting police out on the street, cutting crime, and delivering a new immigration system and I'm sticking by her."
Labour leader Jeremy Corbyn accused the "part-time" prime minister of having "no shame" and repeated his call for an independent inquiry into Ms Patel's conduct.
He demanded to know if Mr Johnson was aware of the complaints about her behaviour when he appointed her home secretary.
The PM hit back by branding Mr Corbyn a "full-time Marxist who has failed to stamp out bullying in his own party".
Labour MP Thangam Debbonaire said it appeared that, with this government, facing allegations of bullying "just get you promoted", whereas those who stand up to it lose their jobs.
Mr Johnson told her he "loathes bullying" but added that he would not take lessons on the issue from Labour, whom he accuses of failing to deal with "systematic bullying" against those who "stick up for the Jewish community".
Sir Philip Rutnam, the Home Office's most senior official, resigned on Saturday alleging Ms Patel's conduct towards staff included "swearing, belittling people, making unreasonable and repeated demands".
He said he now intended to take legal action against the Home Office on the basis of constructive dismissal, alleging that he had been forced out of his job.
The BBC reported on Monday that an official in the Department for Work and Pensions (DWP) received a £25,000 payout after alleging she was bullied by Ms Patel in 2015 during her time as employment minister.
On Tuesday, allegations emerged about Ms Patel's behaviour at a third government department.
The BBC's Newsnight reported that in 2017, Ms Patel - then International Development Secretary - was allegedly accused by officials in her private office of humiliating civil servants in front of others, of putting heavy pressure in emails and of creating a general sense that "everyone is hopeless".
The allegations were reportedly brought to a senior official at the Department for International Development after Ms Patel quit as its secretary of state in 2017.
Mr Corbyn told the prime minister that "if true" the allegations suggest "a shocking and unacceptable pattern of behaviour across three government departments".
The Labour leader's spokesman later said: "We've had government staff contact our office directly with information and allegations about bullying in the Home Office by Priti Patel."
The allegations relate to Ms Patel and another former minister, the spokesman added.
"They are bullying and harassment allegations of government staff and they simply build up the picture that has already accumulated in recent days.
"It is quite clear this is not an isolated allegation by one individual about one incident or one set of incidents."