Priti Patel’s future as the home secretary is being called into question this weekend after the most senior civil servant in her department resigned in a row over her alleged bullying of staff – and then announced he would sue the government for constructive dismissal.

In a move described as “unprecedented” and “extraordinary” by two former heads of the home civil service, Sir Philip Rutnam accused Patel of orchestrating a “vicious” campaign against him and of “shouting and swearing, belittling people, making unreasonable and repeated demands – behaviour that created fear and that needed some bravery to call out.”

Rutnam said: “One of my duties as permanent secretary was to protect the health, safety and wellbeing of our 35,000 people. This created tension with the home secretary and I have encouraged her to change her behaviours.” He said he had been accused of briefing the press against Patel, a claim he said was completely untrue.

“The home secretary categorically denied any involvement in this campaign to the Cabinet Office,” he said. “I regret I do not believe her. She has not made the efforts I would expect to disassociate herself from the comments.”

On Saturday night, Keir Starmer, the favourite to succeed Jeremy Corbyn as Labour party leader, called on Patel to make a statement to parliament on Monday “to explain the allegations made about her own conduct”. He also demanded that the cabinet secretary, Sir Mark Sedwill, launch an immediate investigation into the reasons for Rutnam’s departure, adding: “Over recent weeks, there has been growing evidence of a serious breakdown of government. The prime minister’s handling of the flooding and coronavirus has been woeful, advisers in No 10 are out of control, and attacks on the civil service are growing.”

Dave Penman, general secretary of the FDA – the trade union for senior civil servants, which will back Rutnam’s legal case – said Rutnam had been offered the chance to “resign and go quietly with financial compensation” but had instead chosen to speak out against “attacks on civil servants”. It is expected that Rutnam will demand a public hearing at which he can outline his case against Patel.

Penman added: “The Home Office now needs to find new leadership at a time when it needs stability. Those who engage in anonymous briefings need to bear the responsibility for this destructive behaviour.”

Doubts over the home secretary’s ability to survive come less than three weeks after Sajid Javid’s resignation as chancellor, when he refused to bow to demands from Boris Johnson and the PM’s most senior aide, Dominic Cummings, to sack all his advisers.

Sir Philip must have been driven to the limit and beyond. To resign is serious enough, but then to take legal action is quite extraordinary Lord Kerslake, former civil service head

It also emerged recently that a former adviser to Javid, Sonia Khan, is pursuing a case for unfair dismissal and sex discrimination after she was sacked by Cummings and marched out of Downing Street by an armed police officer last September.

Patel has already resigned once from the cabinet, when in 2017 she had to quit as international development secretary, having failed to be candid about unofficial meetings with Israeli ministers, business executives and a senior lobbyist.

Lord Kerslake, who was head of the civil service from the end of 2011 until September 2014, said he knew Rutnam well and had been involved in appointing him to his first post as permanent secretary at the Department for Transport in 2012.

“For him to do this means he must have been driven to the limit and beyond,” Kerslake said. “To resign is serious enough, but then to take legal action is quite extraordinary. I think it will send bad signals through the civil service. It speaks of some very toxic relationships at the heart of government.”

Kerslake said he would expect Patel to resign if Rutnam won his case for constructive dismissal.

Lord Turnbull, a former cabinet secretary, said such personnel disputes were normally settled quietly behind the scenes. “When there is an issue, a way would normally be found to secure a judicious job swap or some other route would be found to resolve the difficulty,” he said. “For this to have got to this stage is extraordinary and potentially extremely damaging for the civil service.”

When asked if Johnson retained full confidence in his home secretary, a Downing Street official said: “The prime minister has complete confidence in all of his cabinet.”

Labour MP Stephen Doughty, a member of the home affairs select committee, said Patel’s position was now in doubt “following this chaos”. He added: “This utterly extraordinary statement by a senior civil servant is clear evidence of just how dangerously dysfunctional relationships are becoming at the heart of this government – driven by a toxic culture at the very top.”

The tensions between Patel and her permanent secretary surfaced last weekend. Reports suggested she had clashed with senior officials, belittled colleagues and was distrusted by intelligence chiefs. One report suggested Patel had tried to move Rutnam from her department after the rows.

Labour’s Yvette Cooper, the chair of the home affairs select committee, said the row reflected “extremely badly on the government”.

She said: “To end up with one of the most senior public servants in the country taking court action against one of the great offices of state shows a shocking level of breakdown in the normal functioning of government.

“For the home secretary and prime minister to have allowed things to reach this point is appalling, especially at a time when the Home Office faces crucial challenges with rising violent crime, forthcoming counter-terror legislation, new immigration laws, and sensitive negotiations on post-Brexit security cooperation.”