Three months after his triumphant re-election, British Prime Minister Boris Johnson is battling to save one of his top ministers from a bullying scandal that has exposed tensions at the heart of his government.

Interior minister Priti Patel has faced calls to resign over allegations that she mistreated staff in both her present and previous roles -- claims she strongly denies. Johnson on Wednesday offered his full support for Patel, a fellow veteran of the Brexit campaign in the 2016 EU referendum, telling MPs she was doing an "outstanding job".

But the allegations keep on coming, and under pressure, the prime minister launched an internal inquiry into whether Patel has broken the ministerial code of conduct. The row erupted after the top civil servant in the Home Office Patel leads resigned on Saturday, accusing her of involvement in "vicious" briefings against him after he raised concerns about her behaviour.

"I have received allegations that her conduct has included shouting and swearing, belittling people, making unreasonable and repeated demands," Philip Rutnam said.

In an extraordinary televised press statement, he also said he would be suing the government for constructive dismissal -- effectively alleging he was forced out of his job. Patel has strongly denied bullying and one aide accused "dark forces" in the civil service of trying to undermine her.

Supporters point to one explosive newspaper report that intelligence chiefs do not trust Patel, who is in charge of security -- something the government emphatically denied.

Perfect storm

Alex Thomas, from the Institute for Government think-tank, said the row at the Home Office showed an "extreme and extraordinary breakdown in relations at the very top".

It comes as the ministry faces the gruelling challenge of delivering two of Johnson's key promises from the December election. This is to provide 20,000 new police officers, and introduce a new post-Brexit immigration system by the end of the year.

"It is, if not impossible, then very near impossible to deliver everything that it has to do over the next 12 months," Thomas, an expert in the civil service, told AFP. "And distractions like this do take away from the leadership."

Rutnam himself, meanwhile, was under pressure over a forthcoming report into the dismal treatment of so-called Windrush Britons originally from the Caribbean. Thomas added: "It feels to me that the Home Office was a bit of a perfect storm, with personality types and a particularly difficult agenda that created this confrontation."

Will of the people

But the row also broke out against a backdrop of tensions between Downing Street and the permanent, politically neutral civil servants who run the British government. Johnson's senior adviser, Dominic Cummings -- who also worked on the Brexit campaign -- has been open about his desire to shake up what he sees as a slow-moving establishment.

Last month, it was reported that No 10 had drawn up a list of top civil servants it wanted to replace because they were "at odds" with Johnson's team -- Rutnam among them. "People take their cues from Number 10," Thomas noted of the Home Office row.

Patel has strong support among her fellow Conservative MPs, many of whom have long complained about civil servants trying to thwart Britain's exit from the European Union. After Brexit on January 31, some still suspect officials are trying to stymie plans for Britain to forge its own independent path.

"Civil servants are crown servants and, as I am sure they would agree, they really do have to carry out the will of the people," Tory MP Bill Cash said in support of Patel this week.

But there has been resistance even among ministers to Cummings-inspired reforms. Sajid Javid quit as finance minister last month rather than accept changes that would have seen him unable to appoint his own political aides.