A civil servants’ union has written to Boris Johnson seeking assurances Whitehall staff will not be asked to break the law during his manoeuvrings on Brexit.

With Downing Street insisting the Government will not comply with an act designed to avoid a no-deal Brexit, the FDA has written to its members saying senior civil servants might be forced to break the the law and may be prosecuted if they help the Prime Minister defy the will of Parliament.

The letter from FDA general secretary Dave Penman tells Mr Johnson of “increasing consternation” among civil servants.

Mr Penman told Mr Johnson that Brexit had been a “lightning rod for attacks on the civil service”.

Prominent figures who “seek to further their own political agenda” had been prepared to sacrifice public confidence in the service through “baseless allegations of bias” and other attempts to undermine its integrity.

“As Prime Minister and therefore Minister for the Civil Service, civil servants look to you to defend these principles,” Mr Penman wrote, “but increasingly over the last few weeks the greatest concern for civil servants has come from the very office that is meant to be at the frontline of its defence”.

“Whatever political calculations that are being made about how this may play out with groups of the electorate, the suggestion that the government, and by implication the civil service, will be asked to ignore the settled will of Parliament, is causing increasing consternation among civil servants.

“No civil servant should believe there is a conflict between complying with the law and serving the government of the day – and no Prime Minister should place the civil service in such an invidious position.”

He added: “There should be no grey areas when it comes to the duty to uphold the law and abide by the civil service code. Civil servants should not be placed in a position where they are expected to be, or are seen to be, arbiters of the law. Almost unimaginably, the endless speculation emanating from No 10 suggests that is where they may end up.”

Mr Penman said only Mr Johnson could end this speculation.

“I am therefore asking you to categorically and publicly assure the civil service that no civil servant will be asked to breach their obligation under the civil service code to ‘comply with the law and uphold the administration of justice’,” he wrote, in the letter published in the Guardian.

Mr Penman also noted Wednesday’s Scottish court ruling that Mr Johnson’s suspension of Parliament was unlawful would “only add to the uncertainty and increase concerns among civil servants”.