A group of UN human rights experts have appealed to Singapore to halt an execution scheduled this week of a Malaysian who smuggled drugs into the city-state, on the grounds that he has intellectual disabilities.

Nagaenthran K Dharmalingam, 33, is scheduled to be hanged on Wednesday, but the court stayed his execution pending an appeal to be heard on Tuesday. Singapore has some of the world’s toughest laws on drugs.

The court had earlier dismissed an argument that hanging Nagaenthran would violate Singapore’s constitution because he was intellectually challenged.

“We are seriously concerned that, if the appeal is dismissed, he could still be executed imminently,” the experts said in a statement.

The UN experts called on Singapore to commute the death sentence against Nagaenthran, in line with international human rights law.

Nagaenthran was detained in April 2009 for trafficking into Singapore about 42.72g of diamorphine, or pure heroin, which was strapped to his thigh. He was sentenced to death.

His lawyer M Ravi, and activists say Nagaenthran’s IQ was found to be at 69, a level recognised as a mental disability, and he has other disorders that affect his decision-making and impulse control.

Authorities previously said Singapore courts were satisfied that Nagaenthran knew what he was doing when he committed the offence.

The case has attracted international attention including from British billionaire and capital punishment opponent Richard Branson, who called on Singapore to spare Nagaenthran from hanging.

Amnesty International said that while the stay offered a “glimmer of hope”, time was running out for Nagaenthran.

“To meet international standards of fairness, an appeal process must not be rushed, but needs to serve as a meaningful opportunity to reconsider Nagaenthran’s case,” said Rachel Chhoa-Howard, its Singapore researcher.

Prime Minister Ismail Sabri Yaakob had also written to his Singaporean counterpart Lee Hsien Loong seeking leniency for Nagaenthran.