The UN special rapporteur on torture has accused David Cameron of a “cold-hearted ” approach to the migration crisis, warning that plans to scrap the Human Rights Act risk subverting international obligations designed to protect people fleeing persecution.

Juan Méndez said the UK’s intention to replace the act with a British bill of rights was a “dangerous and pernicious” development. Méndez said that the government’s proposals indicated a lowering of protection for people that would leave individuals at risk of cruel, inhuman or degrading treatment and being refused asylum and deported despite facing mistreatment.

He said such a move could contravene Britain’s obligations under international law and set “a very bad example for the rest of the world”, potentially allowing other states to dilute their levels of protection for vulnerable people.

“The problem is that the line between cruel, inhuman or degrading treatment and torture is very difficult to draw. If a country is going to mistreat you, then what is going to stop them torturing you? This would be a dangerous development, a disastrous reading because it would violate the object and purpose of the norm.

“It is not to give governments flexibility in deciding who stays; it is to protect people from torture and ill-treatment. You could call this a bad-faith interpretation,” he said.

The government is due to unveil its detailed plan for the British bill of rights soon, a move that ministers say will give the UK supreme court supremacy over the European court of human rights.

Méndez said mooted changes making it easier to return or deport people even if they faced persecution sent a “pernicious” message as Europe remained divided over its response to the migration crisis. On Friday, the most senior Foreign Office official, Sir Simon McDonald, said human rights were no longer a UK “top priority”, and ministers were prioritising support for trade deals over tackling injustice abroad.

Méndez said the timing of the new bill of rights, in the middle of a migration crisis, would seem distasteful to many international observers.

“There are so many people in need of protection that this would read as an ungenerous and cold-hearted way of dealing with a crisis,” he said.

One effect of the potential changes might be allowing the UK to return people to countries such as Sri Lanka - where there is evidence that the security forces torture those deported - while avoiding the issue of torture between respective governments.

“Nobody is saying the UK should not have relations with countries like Sri Lanka, having relations is one thing, protecting nations from scrutiny while making the extra step to send people back to countries where there is the obvious risk they will be tortured is wrong, politically, legally and morally,” said Mendez.

Sonya Sceats, director of policy and advocacy at the London-based group Freedom from Torture, which treats and rehabilitates survivors, said: “When the British public is expressing so much sympathy for those forced to flee torture and other atrocities, how could the government contemplate weakening human rights protection for them here?

“Survivors of torture we have spoken with struggle to accept that this country with its proud tradition of liberty could be about to turn its back on the fundamental principle that human rights are universal. It would be a betrayal of our history and our global status as a beacon of hope.”

The Conservatives promised to repeal the Human Rights Act in their election manifesto, although Scotland’s first minister, Nicola Sturgeon, recently said it was “inconceivable” that the Scottish government would consent to Tory plans to scrap the Human Rghts Act, a move that would be a “monumental mistake”.

Justice minister Dominic Raab said: “This is idle speculation. We will set out our proposals for full consultation in due course, and it’s irresponsible for any UN official to criticise our plans without knowing what they are.”