The top civil servant in the Home Office has resigned and said he intends to sue the government for constructive dismissal.
Sir Philip Rutnam said there had been a “vicious and orchestrated” campaign against him in Home Secretary Priti Patel’s office.
Reported tensions between the pair included claims she mistreated officials – which she has denied.
The BBC’s Laura Kuenssberg said Sir Philip’s move was “highly unusual”.
Our political editor added: “I can’t remember a senior public official taking a step like this.”
Sir Philip, who has had a career spanning 33 years, said he believed his experience was “extreme” but part of a “wider pattern” in government.
He said he received allegations that Ms Patel’s conduct towards employees included “shouting and swearing” and “belittling people”.
It comes days after the home secretary and Sir Philip released a joint statement saying they were “deeply concerned” by various “false allegations” made about Ms Patel.
Allegations the pair dismissed included reports that Ms Patel, who has been home secretary since Boris Johnson became prime minister, bullied her staff and was not trusted by MI5 bosses.
Ms Patel has not yet commented on Sir Philip’s resignation.
In a statement given to BBC News, Sir Philip said: “In the last 10 days, I have been the target of a vicious and orchestrated briefing campaign.”
He said allegations he had briefed the media against the home secretary was one of many “completely false” claims against him.
“The home secretary categorically denied any involvement in this campaign to the Cabinet Office.”
He added: “I regret I do not believe her. She has not made the efforts I would expect to dissociate herself from the comments.”
Sir Philip said he had attempted a “reconciliation” with Ms Patel but that she had “made no effort to engage with me to discuss this”.
The FDA union for senior public servants said Sir Philip’s resignation was a consequence of people making anonymous claims about those “who are unable to publicly defend themselves”.
FDA general secretary Dave Penman said the “cowardly practice” was “ruining lives and careers” as well as diverting resources.