Boris Johnson's controversial decision not to sack Priti Patel for bullying could be challenged in the courts.
Lawyers acting for the senior civil servants' union the FDA issued a pre-action notice to Downing Street accusing the Prime Minister of acting unlawfully by clearing the Home Secretary of misconduct under the Ministerial Code – and overruling the findings of his own independent standards adviser.
If the legal bid succeeds, it could see Mr Johnson's decision struck down by judges.
Outlining the potential court fight, FDA union general secretary Dave Penman said in an internal message to members, seen by the Mirror: “We now find ourselves in a position where the Prime Minister has exercised his prerogative power in relation to the Ministerial Code and made a perverse decision which if unchallenged would leave those civil servants very exposed.
“Given the consequences for those civil servants who were bullied and the impact across the service, we asked our legal advisers to consider whether the decision of the Prime Minister could be challenged in the courts.
“As a result, we have now begun the legal process which may ultimately result in a Judicial Review application, seeking to quash the Prime Minister’s decision.
“As is required in these circumstances, we have issued a pre-action protocol letter setting out where we believe the Prime Minister erred in law, asking him to reverse the decision and also seeking further details of any facts he relied on in making his decision.
“Once we have a response to this letter, we will review the position, as you would expect, before any final decision is made to proceed to Judicial Review.
“This is not a course of action we have taken lightly.”
Ms Patel faced calls to quit after the results of an inquiry were revealed last month.
The Home Secretary was found to have shouted and sworn at officials, according to the findings of a probe.
But she was allowed to cling to her job after Mr Johnson rejected advice from standards adviser Sir Alex Allan that she broke the Ministerial Code.
Sir Alex quit after his verdict was rebuffed.
In his message to members outlining the next steps of the legal fight, Mr Penman added: “This is not a simple challenge and raises issues regarding the scope for challenge of the Prime Minister’s prerogative powers.
“We are not seeking to interfere with his power over ministerial appointment or dismissal; the consequences of breaching the Ministerial Code are rightly a matter for the Prime Minister alone.
“Our challenge rather centres on his decision that the Home Secretary did not breach the Ministerial Code, despite the facts that were established and which he did not challenge.”
Speaking after she was allowed to keep her job, Ms Patel told the BBC "there are no excuses" for her conduct, admitting: "I've clearly upset people."
She added: "I am sorry if I have upset people in any way whatsoever. That was completely unintentional."