Priti Patel is facing the possibility of being questioned before a 10-day employment tribunal hearing next September after lawyers for her former permanent secretary Sir Philip Rutnam pushed forward with a claim for constructive dismissal. The home secretary, her department, and Downing Street are also being asked to disclose all correspondence related to Rutnam’s departure following claims that he was forced out of his job for intervening in her alleged bullying of fellow civil servants. A schedule for an employment tribunal has been sent to the Home Office’s lawyers which names the home secretary as the respondent, the Guardian understands. Both parties have been asked to exchange information on a voluntary basis by January. Witness statements are expected to be exchanged by July and the 10-day hearing is scheduled to take place from 20 September. The development will increase pressure on Patel, who has denied all allegations against her, and on Boris Johnson, who has publicly backed his home secretary. The government will resist any attempts to force Patel or Johnson to appear before the tribunal as witnesses, it is understood. It is the first time that a secretary of state has been pursued to an employment tribunal by a former permanent secretary, the most senior civil servant in a government department. Patel is still the subject of a seven-month Cabinet Office inquiry, which has yet to be signed off by the prime minister, into her behaviour. Rutnam’s case is expected to focus on his claims that in late 2019 and early 2020, he challenged Patel’s alleged mistreatment of senior civil servants in the Home Office, and that he was then hounded out of his job through anonymous briefings. Reports claimed that a senior Home Office official collapsed after a fractious meeting with Patel. She was also accused of successfully asking 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. 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, the pessimistic donkey from Winnie the Pooh. 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. He is represented by Gavin Mansfield QC, the head of Littleton Chambers and an employment law specialist, and Clive Howard from the law firm Slater & Gordon. His legal case is being supported by the FDA union. Dave Penman, the union’s general secretary, claimed last week that the delay in publishing the seven-month bullying inquiry’s findings has eroded trust between ministers and senior civil servants. Rutnam is making a claim of “protected disclosure” under whistleblowing laws which 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. Patel is expected to be asked to appear as a witness at the hearing by Rutnam’s lawyers. Sir Mark Sedwill, the then cabinet secretary, could also be asked to appear as Rutnam’s then line manager. The question of who will be a witness will be settled by the tribunal shortly before the hearing. The government and Rutnam did not respond to a request for a comment.