Human rights campaigners have challenged the prime minister in the high court, accusing her of abandoning the longstanding principle that members of the government should be bound by international law.

In a hearing in the court of appeal on Wednesday, campaigners from the Gulf Center for Human Rights (GCHR) argued that ministers had abandoned their commitment to abide by international law after quietly rewriting the ministerial code in 2015.

The code has been in existence since 1997 and sets out the standard of conduct expected by ministers.

The previous code, issued in 2010, said there was 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”.

In the current version the sentence has been edited to say only that there was an “overarching duty on ministers to comply with the law and to protect the integrity of public life”.

Critics have said changes to the code had far-reaching implications for the UK and its relationship with the rest of the world.

GCHR brought the case against Theresa May and the Cabinet Office minister, David Lidington, after the Guardian revealed the edits to the ministerial code.

Key issues affected by the change could include decisions about whether to go to war or use military force, any decision made by an international court about the UK and any laws not incorporated into English law, such as human rights legislation and the Geneva conventions, lawyers have said.

While there was no dispute about the fact that ministers deleted the key clause from the 2015 version of the ministerial code Jonathan Crow QC, acting for the government, said there were no concrete examples of ministers no longer abiding by international law since removing the clause.

However, Jason Coppel QC, for GCHR, told the court that the government’s change of the ministerial code was “substantive”.

“This can make a difference to individual cases in the future,” he said. “They [ministers] have a connection with parts of the world where international obligations may be very important, such as Syria and Yemen.”

When the Guardian revealed the deletion, the Cabinet Office denied there was any intention to weaken international law and the administration of justice by omitting the phrases from the new code.

But lawyers for GCHR accused the then prime minister David Cameron of making changes to the code in secret, behind closed doors and without any public scrutiny despite its constitutional significance.

David Cameron was the prime minister when the code was edited in 2015 to omit the reference to international law. Photograph: Peter Nicholls/Reuters

Sue Willman, a partner at Deighton Pierce Glynn which is representing GCHR, said: “International law is vital not just for our government to be held to account but to ensure that their decisions do not suggest complicity with oppressive governments around the world.”

Melanie Gingell, a member of the GCHR advisory board, said: “Over the past few years, the UK government has sought not only to downplay its own human rights obligations but also to wilfully ignore the violations of their allies in the Gulf. These are states that are systematically abusing their own people.

“The ministerial code is vital to ensure that decisions by UK ministers do not contribute to those abuses. The phrase deleted underpinned the government’s respect for human rights and international law on everything from torture, freedom of speech, respect for women’s and children’s rights to fighting corruption and bribery.”

Judgment has been reserved.