Singapore's Court of Appeal has acquitted a convicted drug trafficker on death row of two capital drug charges and sentenced him to eight years' jail instead for drug possession.

Muhammad Nabill Mohd Fuad, 33, was convicted in the High Court in 2018 of two charges: the first, possessing 63.41g of heroin for trafficking and the other, possessing 2,251.9g of cannabis for trafficking.

In his appeal to the apex court, Nabill, defended by lawyers Andre Jumabhoy and Priscilla Chia, did not dispute that he possessed the heroin and knew what it was.

But the sole issue was whether he possessed the drug for trafficking, his lawyers argued.

On the second charge, Nabill claimed he did not know he had cannabis, neither did he intend to traffic in cannabis.

The Court of Appeal, in reviewing the proceedings, was troubled by the prosecutors' failure to disclose to the defence the statements of four people, all of whom were material witnesses who could have confirmed or contradicted Nabill's defence.

Also, the witnesses were not called to rebut his defence, if their accounts of the events supported the prosecution's case.

This was an issue of "central importance", said the court comprising Chief Justice Sundaresh Menon and Judges of Appeal Judith Prakash and Steven Chong.

"Given the prosecution's overarching duty of fairness, we were troubled that statements recorded from four such witnesses were not disclosed to the defence... neither were these witnesses called by the prosecution to rebut the appellant's defence if, indeed, their accounts of the events supported the prosecution's case," said Chief Justice Menon on the court's behalf.

In judgment grounds issued on Tuesday , the court agreed with the parties that the prosecution is duty-bound to disclose a material witness' statement to the defence, regardless of whether it was favourable, neutral or adverse to the accused person.

Also, while the prosecution had no legal duty to call a material witness, its failure to do so might mean it had failed to provide sufficient evidence to discharge its burden, it said.

The court also said the failure to call a material witness might entitle the court to infer the witness' evidence would have been unfavourable to the prosecution .

6 points to bear in mind: Court

The Court of Appeal laid out six points that all judges, prosecutors and defence counsel must and ought to apply to every criminal case they deal with. These are: (a) prosecution's burden to prove its case beyond reasonable doubt; (b) strict rules of criminal procedure which require the prosecution to first prove a prima facie case, before the defence could be called or even invited to set out material aspects of its position; (c) prosecution's task to present its case at the trial; (d) reasons for a judge to exercise considerable restraint in intervening while the accused person is giving his evidence-in-chief; (e) tendency for an accused to present himself to the judge in as agreeable a manner as possible so as not to upset him or her; (f) need for a judge to refrain from asking leading questions generally. K.C. Vijayan

In Nabill's case, these material witnesses could have contradicted his narrative of the events, the court added.

The court set aside the first charge and convicted him on a reduced charge of drug possession, to which he admitted.

It acquitted him of the second charge as his defence had rebutted the presumption of knowledge. And since knowledge had not been established, he could not be held guilty even of an offence of possessing cannabis.

The court rejected his claim of excessive judicial interference during the High Court hearing.

"Nonetheless, we take this opportunity to examine and reiterate the need for judges to exercise especial prudence, caution and restraint in criminal proceedings, where the consequences of excessive judicial interference on an accused person's life and liberty may be severe indeed," said the Chief Justice.

Nabill had claimed the judge took over "the conduct of the questioning of witnesses", among other things.

About 30 of his family members, relatives and friends were present in court, many of whom teared up on hearing the verdict.

Mr Jumabhoy said that this is an important case "as it goes towards levelling the playing field between the prosecution and defence in the search for the truth".