Boris Johnson’s decision to back Priti Patel and disregard the findings of his adviser on ministerial standards that the home secretary had bullied staff can be challenged in court, a hearing has ruled.

The FDA union has been granted permission to launch a judicial review of the prime minister’s decision, despite being challenged by the government in the high court.

It means the union will be able to argue at a future court hearing that Johnson’s interpretation of the ministerial code should be subject to the rule of law.

Responding to Tuesday’s decision, which is the first step toward a judicial review, the union’s general secretary, Dave Penman, said: “The ministerial code is the only means by which civil servants can raise complaints against the conduct of ministers and it is vital that decisions on this are subject to the rule of law.

“Ministers should be held to the same standards of conduct as civil servants. We welcome the opportunity now granted to argue that point fully that the prime minister erred in his interpretation of the ministerial code when deciding that the home secretary did not break the code.”

Sir Alex Allan left his advisory role in Downing Street after Johnson contradicted his advice by judging that Patel did not breach the rules, despite being found to have bullied staff.

The FDA’s lawyers asked the high court on Tuesday to grant permission for a full hearing of the judicial review claim.

In an investigation into Patel’s behaviour published in November, Allan found she had not always treated civil servants with “consideration and respect”.

He concluded that her behaviour, which was said to include some occasions of shouting and swearing, met the definition of bullying adopted by the civil service.

Ministers are usually expected to resign if they breach the code, but Johnson, who is its arbiter, judged that Patel had not fallen foul of the rules.

Allegations against Patel emerged in March 2020 after the resignation of Philip Rutnam, the former Home Office permanent secretary, over what he described as a “vicious and orchestrated campaign” against him for challenging the alleged mistreatment of civil servants.

According to reports, a senior Home Office official collapsed after a fractious meeting with Patel, who is understood to have successfully asked for another senior official in the department to be moved from their job.

Further allegations emerged against Patel in the aftermath. An official in the Department for Work and Pensions (DWP) received a £25,000 payout after she alleged that Patel, who was employment minister at the time, had bullied her in 2015. The DWP did not admit liability and the case did not come before a tribunal.

Officials in Patel’s private office at the former Department for International Development allegedly accused her of humiliating civil servants in front of others while a minister in 2017.

Patel previously issued an “unreserved, fulsome apology” and said there were “no excuses” for what happened.