N Surendran, who is adviser to rights group Lawyers for Liberty, said the execution was an “unlawful and brutal act, carried out in breach of due process and in defiance of the appeals made by neighbouring Malaysia”.

“Late last night, the Singapore President’s Office delivered a letter to Prabu’s family in response to their clemency petition. The letter stated that ‘the clemency process has concluded’ and ‘we are unable to accede to your request’.

“This rejection is unlawful under the Singapore constitution as there is no bar to the filing of a fresh clemency petition. It is a flagrant breach of due process for the Singapore president to reject the family’s clemency petition without even considering it.”

In a statement, Surendran questioned the “unholy haste” with which Prabu had been executed, reiterating earlier concerns that the family was given less than a week’s notice to prepare for his death.

“Was this an underhanded method by Singapore authorities to execute secretly and prevent representations being made or court applications being filed?”

He noted that even ministers from Malaysia’s Cabinet had spoken directly to Singapore’s leaders, to no avail.

“It is disturbing that the Singapore government is prepared to disregard basic processes in their rush to execute this young Malaysian,” he added.

Prabu, 31, was found guilty of trafficking drugs across the causeway in 2014.

Surendran had questioned the circumstances surrounding Prabu’s conviction, noting that the drugs were found in a vehicle driven by another person.

He also said that confessions obtained against Prabu were made under duress.

Confirming Prabu’s death this morning, Surendran said the family had claimed his body and would be cremating his remains today.

“Prabu’s parents and siblings are devastated by the execution of their son and brother.

“They wish to thank the Malaysian government, and everyone on both sides of the causeway, for their support and prayers over the past few difficult days.”