Boris Johnson has been warned not to force civil servants to break the law by ignoring Parliament.
The FDA trade union has written to the prime minister demanding assurances that staff will not be asked to act illegally or break the civil service code after MPs passed a law aimed at making it harder for the Government to leave the EU without a deal.
In the letter, seen by The Guardian, the union's general secretary Dave Penman hit out at "tactical political game playing" and said there was increasing concern in Whitehall over ministers' public comments on whether the government will abide by the legislation.
Penman, who has previously told CSW that the civil service must not be dragged into a political fight over Brexit, urged the prime minister to make clear that officials will not have to do anything that undermines their "professional integrity".
"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," the union boss said.
"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."
Foreign secretary Dominic Raab this weekend suggested the government would "test" the legislation before implementing it, while ministers have repeatedly said they will be seeking an extension to Article 50 – a key demand of the law if no Brexit agreement can be found by mid-October.
Penman added: "The endless speculation that government will refuse to implement an Act of Parliament may serve short term political interest, but as prime minister your responsibilities go beyond tactical political game playing. As prime minister, your duty is to ensure that the impartiality and integrity of the civil service is protected.
"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."
And he said: "Only you, as prime minister, can end this speculation. I am a 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'."
Former attorney general Dominic Grieve, one of the authors of the rebel Brexit bill, told The Guardian that he understood why civil servants were "getting increasingly worried by the extraordinary words, statements and behaviour coming from No 10 Downing Street".
"Our constitution depends upon people observing rules of honesty and integrity and we seem to be losing them very quickly at present," the ex-Tory MP said.
And Lord Kerslake, the head of the civil service from 2011 to 2014 , warned: "It is quite extraordinary that there is even a question of putting civil servants in a position where they could break the civil service code and that ministers are seriously contemplating breaking the law."
“This development confirms the very serious place we have now reached.”
The crossbench peer said officials concerned about the government's stance could be forced to seek a ministerial direction, the formal process by which a civil servant raises their objection to an order from a minister.