Singapore is set to hang a man for trafficking cannabis, in the city-state's latest controversial execution.

Activists say Tangaraju Suppiah was convicted on weak evidence. Authorities say he received due process, and have scheduled his execution for Wednesday.

It follows a high-profile execution last year of a mentally disabled man over a drugs charge.

Singapore has some of the world's toughest anti-drug laws, which it says are necessary to protect society.

In recent days his family members and activists delivered letters to Singapore's president Halimah Yacob in a last-minute plea for clemency, while British billionaire Sir Richard Branson has called for a halt of the execution and a review of the case.

"I know that my brother has not done anything wrong. I urge the court to look at his case from the beginning," Tangaraju's sister, Leela Suppiah, told reporters at a news conference.

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