British tycoon Richard Branson Monday urged Singapore to halt the imminent execution of a man sentenced to death over one kilogram of cannabis, saying it "may be about to kill an innocent man".

Singaporean Tangaraju Suppiah, 46, is scheduled to be hanged on Wednesday, the city-state's Central Narcotics Bureau has said.

"Tangaraju was actually not anywhere near these drugs at the time of his arrest. This was largely a circumstantial case that relied on inferences," Branson, who is a member of the Geneva-based Global Commission on Drug Policy, wrote on his blog.

"Killing those at the lowest rungs of the illicit drug supply chain... is hardly effective in curbing an international trade worth hundreds of billions every year," he said, adding he hoped authorities would review the case.

In many parts of the world – including in neighbouring Thailand – cannabis has been decriminalised and rights groups have been mounting pressure on Singapore to abolish capital punishment.

The Asian financial hub has some of the world's toughest anti-narcotics laws and insists the death penalty remains an effective deterrent against trafficking.

Tangaraju was convicted in 2017 of "abetting by engaging in a conspiracy to traffic" 1,017.9g of cannabis, twice the minimum amount that merits the death sentence under the city-state's tough drug laws.

He was sentenced to death in 2018 and the Court of Appeal has upheld his sentence.

Tangaraju's family on Sunday also pleaded for clemency and urged a retrial.

If carried out, it will be Singapore's first execution in six months and the 12th since last year.

"Singapore is an otherwise wonderful country, so it's very sad to see some of its policies harking back to colonialism, and even reminiscent of medieval times," Branson said.

Branson and rights groups say Tangaraju never handled the drugs himself.

Prosecutors have said two mobile phone numbers he owned were used as contacts.

Singapore resumed executions in March 2022 after a hiatus of more than two years.

Among those hanged was Nagaenthran K Dharmalingam, whose execution sparked a global outcry, including from the United Nations and Branson, because he was deemed to have a mental disability.