Priti Patel is facing legal action under whistleblowing laws after her former permanent secretary Sir Philip Rutnam lodged an employment tribunal claim on Monday saying he was forced from his job for exposing her bullying behaviour.

Rutnam claims he was constructively dismissed from his role as Home Office permanent secretary after informing the Cabinet Office that Patel had belittled officials in meetings and made unreasonable demands on staff.

The development will increase pressure on Patel, who has denied claims that she bullied civil servants in three government departments, and on Boris Johnson, who has publicly backed his home secretary. She is also the subject of a Cabinet Office inquiry into her behaviour.

It is the first time that Rutnam has said he will make a claim of “protected disclosure” under whistleblowing laws and the first time that a secretary of state has been sued by a former permanent secretary.

Another consequence of a whistleblowing claim means that instead of a ceiling of £85,000 on Rutnam’s claim, there will be no upper limit to any damages he might receive.

Announcing the start of the legal action, the general secretary of the civil service union the FDA, Dave Penman, told the Guardian: “This morning Sir Philip, with the support of his legal team and the FDA, submitted a claim to the employment tribunal for constructive dismissal and whistleblowing against the home secretary. Sir Philip will not be making any further comment at this time.”

Rutnam’s claim is expected to focus on Patel’s treatment of senior civil servants in the Home Office. According to reports, a senior Home Office official collapsed after a fractious meeting with Patel. Patel is understood to have successfully asked for another senior official in the department to be moved from their job.

Rutnam subsequently wrote to all senior civil servants in the department highlighting the dangers of workplace stress. He also made clear that they could not be expected to do unrealistic work outside office hours.

Sir Philip Rutman addressing the home affairs select committee in February. Photograph: AFP via Getty Images

After a report in the Times highlighted tensions between Rutnam and Patel, sources close to Patel were quoted in several newspapers as saying that Rutnam should resign. An article in the Times said he should be stripped of his pension, another in the Telegraph said he was nicknamed Dr No for negative ideas, while one in the Sun compared him to Eeyore.

At that time the prime minister’s official spokesman said Johnson had full confidence in the home secretary and in the civil service, though the same guarantee was not given to Rutnam specifically.

Rutnam resigned on 29 February, reading a hastily arranged statement outside his home. In the statement, he accused Patel of orchestrating a “vicious” campaign against him, of lying about her involvement in it and of creating a climate of fear in her department.

Rutnam is expected to be represented by Gavin Mansfield QC, the head of Littleton Chambers and an employment law specialist. He will be supported by Clive Howard from the law firm Slater & Gordon.

Patel would be expected to appear as a witness, along with her special advisers, and the government would be asked for full disclosure of emails and correspondence between Patel, her staff, the Cabinet Office and No 10.

Bruce Carr, a leading QC and former government adviser on employment legislation, said the significance of a whistleblowing claim was that the compensation would be uncapped, unlike an ordinary unfair dismissal claim.

“Rutnam will have to show that he made a ‘protected disclosure’ – such as a disclosure of information showing there has been a breach of a legal obligation. This could include an allegation that others had been bullied, which would amount to a breach of their contract of employment. It could also include an allegation that a minister had breached the ministerial code,” he said.

The ministerial code states: “There must be no bullying and no harassment.”

A Cabinet Office inquiry into the allegations of bullying and Rutnam’s resignation is being carried out by Sir Alex Allan, the prime minister’s independent adviser on ministers’ interests. He will decide whether Patel broke any rules. However, the final say rests with Johnson, who can overrule Allan and keep his findings secret.

Despite holding one of the four great offices of state during a national pandemic, Patel has rarely been seen in public since the allegations of bullying emerged in February. She has made one appearance at the daily Downing Street coronavirus briefing, and was criticised after saying she was sorry if people felt there had been a failure to supply sufficient personal protective equipment to NHS staff.

A government spokesperson said: “We do not comment on ongoing legal proceedings.” Patel’s office has been approached for a comment.