Nagaenthran K Dharmalingam, 33, was due to be executed by hanging on Wednesday for trying to smuggle less than 43 grams of heroin into the country.

The court granted a stay of the execution after Nagaenthran's defence lawyer, M Ravi, argued that capital punishment for a mentally disabled person was a violation of Singapore's constitution.

Ravi's application was dismissed but the stay on the execution was granted "pending the hearing of the appeal to the Court of Appeal," Singapore's top court, the lawyer said on Facebook.

The appeal is scheduled to be heard via videoconference on Tuesday.

If this fails, the stay will be lifted and Nagaenthran could be executed as scheduled.

Narcotics officers found a small bundle of heroin strapped to Nagaenthran's left thigh at a checkpoint more than a decade ago.

He was sentenced to death in November 2010 under the country's strict drug laws.

A previous appeal to reduce the penalty to life in prison failed and a final push for presidential clemency was rejected last year.

Death penalty opponents say Nagaenthran's IQ was disclosed during a trial at the High Court as 69, a level that is internationally recognised as an intellectual disability.

But the court ruled that Nagaenthran knew what he was doing and upheld the death sentence.

Nagaenthran's family lives in Ipoh in northern Malaysia.

Activist Kirsten Han, who helped his mother, two younger siblings and cousin arrange travel to Singapore, told the Associated Press that they have been allowed daily prison visits since arriving last week.

"The main thing that I've heard from Nagen's younger brother is that Nagen has been disoriented, he doesn't quite make eye contact, and goes between periods of lucidity and confusion," Han said.

"His brother even doubts that he fully grasps that his execution date is so close," she added.

Nagaenthran's older sister, Sarmila Dharmalingam, who didn't make the trip, said she briefed Nagaenthran about the court outcome during a phone conversation on Monday and that the family held out hope for a positive result.

He thanked her but didn't say much more, she said.

Dobby Chew of the Anti-Death Penalty Asia Network expressed disappointment that the High Court had dismissed the merits of Ravi's argument.

Chew said he hopes Singapore will give adequate time for the appeal.

ADPAN and other human rights groups including Amnesty International and Human Rights Watch have called the execution of an intellectually disabled man inhumane and a violation of international law.

Legal experts, activists and human rights groups in Malaysia have called for Nagaenthran's execution to be halted.

Malaysian Prime Minister Ismail Sabri Yaakob also reportedly asked for leniency "purely on humanitarian grounds" in a letter addressed to Singaporean Prime Minister Lee Hsien Loong, dated November 3.

On Sunday, the European Union delegation to Singapore, along with the diplomatic missions of Norway and Switzerland, called for the execution to be halted.

British business magnate Richard Branson also called for Nagaenthran's life to be spared.

"It's almost always the most vulnerable people, people struggling to make ends meet, immigrants in need of money, that are roped into criminal schemes, unable to defend themselves when caught and facing the court," Branson said in a statement on Monday.

"It's impossible not to see the extent to which inequality, poverty and the death penalty are linked," he added.

Singapore's Ministry of Home Affairs said Nagaenthran has been accorded full due process under the law.

"The High Court had assessed the evidence of the psychiatrists, including the evidence of a psychiatrist called by the defence, on behalf of Nagaenthran, who agreed in court, that Nagaenthran was not intellectually disabled," the ministry said in response to media queries on Friday.

"Nagaenthran considered the risks, balanced it against the reward he had hoped he would get, and decided to take the risk," it added.

The last execution in Singapore was in 2019.

Australian Associated Press