The intervention comes after the decision by the prime minister last year to disregard the conclusions of an internal investigation into the home secretary’s behaviour written by his independent ethics adviser.
The summary of his findings said that Ms Patel’s behaviour, which was said to include some occasions of shouting and swearing, had “been in breach of the ministerial code, even if unintentionally”.
As the ultimate arbiter of the ministerial code, Mr Johnson, however, judged that the code was not breached and said the government considered the matter “closed”. He also urged Conservative MPs to “form a square around the Pritster”.
But in a statement on Friday, the FDA union said it could not allow the to “simply let this issue rest”, as it announced legal action to “overturn” the prime minister’s decision in November.
General secretary Dave Penman said: “Civil servants should expect to work with ministers without fear of being bullied or harassed. Warm woods from the prime minister have no meaning if the process for addressing those behaviours has been fundamentally undermined by his actions.”