The Government is facing legal action from a human rights group after the ministerial code of conduct was amended to remove a reference to the duty to comply with international law.
Rights Watch UK has written to the Government asking for the code - which sets out the standards of conduct expected from ministers - to be restored to its previous version.
The group's director, Yasmine Ahmed, said the change, introduced without fanfare earlier this month, was "seriously concerning".
The new version of the code published by the Cabinet Office states that ministers have an "overarching duty ... to comply with the law", but omits a passage in the earlier 2010 version which added "including international law and treaty obligations".
Rights Watch UK said it was challenging the Government's claim that the amendment does not reflect a substantive change in the code.
The group said the amendment was "particularly worrying at a time when the UK Government is expanding its use of lethal force abroad in circumstances that are legally questionable and when they have made it very clear of their intention to water down the Human Rights Act".
Ms Ahmed said: "For the Government to erase from the ministerial code the starting presumption that its ministers will comply with international law is seriously concerning. It evidences a marked shift in the attitude and commitment of the UK Government towards its international legal obligations.
"There are numerous obligations at international law, including, for instance, those obligations governing the use of force set out in the UN Charter, which have not necessarily been incorporated into domestic law but which it is only proper that ministers should be explicitly required to comply with when exercising their powers."
Ms Ahmed said Human Rights UK's letter to the Government said the code should be restored to a version which made clear that it was "absolutely necessary to ensure that ministers are required to comply with international law in all decisions that they make on behalf of the British public".
The new version of the code was published on the Cabinet Office website earlier this month, accompanied by an explanatory note drawing attention to another change - relating to ministers' contacts with the media - but not to the removal of the reference to international law.