The prime minister is facing a direct legal challenge over changes to the code which sets out rules and standards for ministers' conduct.
A new version of the ministerial code omits a previous reference to ministers being bound by international law.
Senior lawyers claim this could affect the UK's international position and relationship with other states.
The Cabinet Office said the code was very clear on the need to comply with the law, including international law.
Some leading lawyers believe the change could loosen ministerial respect for the rule of law and affect decisions about declaring war or using military force such as drones in Syria.
It could also reduce respect for judgements by international courts on, for example, contentious human rights issues, they add.
By Clive Coleman, BBC legal correspondent
To some this may seem like lawyers arguing over semantics. What is the meaningful difference between the ministerial code referring to a duty to comply with the law "including international law and treaty obligations" or simply the duty to comply with "the law"?
Surely "the law" encompasses all domestic and international law? That is certainly the government's position. However, this dispute focuses on the exercise of prerogative powers by ministers.
These include the power to use force, for instance, in ordering drone strikes in Syria. They are not set down, defined and limited specifically in UK statutes, and are instead governed by international law and treaty, such as the use of force set out in the UN Charter.
It is for that reason that some leading lawyers regard the specific reference to international law in the code as critical.
Paul Jenkins, former head of the government legal service, said it was "disingenuous" to dismiss the changes as "as mere tidying up".
He told the Guardian: "Whether the new wording alters the legal obligations of ministers or not, there can be no doubt that they will regard the change as bolstering, in a most satisfying way, their contempt for the rule of international law."
Campaign group Rights Watch UK said it was challenging the government's claim that the amendment to the code does not reflect a substantive change.
The previous ministerial code, issued in 2010, stressed an "overarching duty on ministers to comply with the law including international law and treaty obligations and to uphold the administration of justice and to protect the integrity of public life".
A new draft simply refers to a duty to comply with "the law and to protect the integrity of public life".
Philippe Sands QC, a professor of law at University College London, told the Guardian the change was "shocking" and "another slap to Magna Carta and the idea of the rule of law".
Also writing in the Guardian, former legal adviser to the Foreign and Commonwealth Office Frank Berman QC said it was "impossible not to feel a sense of disbelief at what must have been the deliberate suppression of the reference to international law" in the new version of the code.
A formal letter, seen by the BBC, is being sent to Prime Minister David Cameron by Rights Watch making it clear it wants the original wording reinstated.
Rights Watch director Yasmine 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."