Singapore's top court on Tuesday postponed the imminent execution of a Malaysian man -- whom campaigners say is mentally disabled -- after he tested positive for Covid-19, with his family hailing a "miracle".

Nagaenthran K. Dharmalingam was arrested in 2009 for trafficking a small amount of heroin into the city-state, which has some of the world's toughest drugs laws. He was sentenced to death the following year.

He was scheduled to be hanged on Wednesday after losing a series of appeals, despite mounting international outrage and supporters' claims his intellectual disability means he is incapable of making rational decisions.

The Court of Appeal was due to hear a last-ditch appeal Tuesday, but campaigners expected the challenge to be rejected, paving the way for the execution to proceed as scheduled.

However, Judge Andrew Phang Boon Leong announced Nagaenthran had contracted Covid-19, meaning the appeal could not go ahead and the execution was stayed.

He cited "logic, common sense and humanity" in deciding to delay proceedings.

"It's something like a miracle," Nagaenthran's sister Sarmila told AFP from the northern Malaysian city of Ipoh.

"We are happy, we are quite relieved."

But she acknowledged the "relief is not permanent" as the appeal will still proceed at some point. No new date has been set for the case yet.

Sarmila said that Nagaenthran, who appeared briefly in court Tuesday before proceedings were postponed, was not ill and did not have any Covid-19 symptoms.

His lawyer, M. Ravi, meanwhile told reporters that he had been "saved by none other than the divine force".

Nagaenthran K. Dharmalingam was sentenced to death in 2020 for trafficking a small amount of heroin into Singapore
Nagaenthran K. Dharmalingam was sentenced to death in 2020 for trafficking a small amount of heroin into Singapore AFP / Mohd RASFAN

There has been a growing outcry at the case, and a group of United Nations human rights experts on Monday became the latest to express opposition to executing someone with an intellectual disability.

"Resorting to this type of punishment to prevent drug trafficking is not only illegal under international law, it is also ineffective," they said.

The European Union has called for Nagaenthran's sentence to be commuted and Malaysia's prime minister has written to his Singaporean counterpart urging a delay in the execution.

British billionaire Richard Branson, a long-standing opponent of capital punishment, has also added his voice to calls to give the Malaysian a reprieve.

Such cases "illustrate why the death penalty is broken beyond repair", he said, adding that sparing Nagaenthran's life would be "the just and fair thing to do".

An online petition calling for his death sentence to be commuted has garnered almost 75,000 signatures.

If the execution does eventually go ahead, it will be the first since 2019 in Singapore, which defends its use of capital punishment as an effective deterrent against crime.

Nagaenthran was arrested at the age of 21 after a bundle of heroin weighing around 43 grams -- equivalent to about three tablespoons -- was found strapped to his thigh as he sought to enter Singapore.

Supporters say he has an IQ of 69 -- a level recognised as a disability -- was struggling with an alcohol problem, and was coerced into committing the crime.

But Singapore's home affairs ministry has defended the decision to press ahead with the hanging, saying that legal rulings had found he "knew what he was doing" at the time of the offence.