The United Nations Human Rights Office pressed Singapore on Tuesday to "urgently reconsider" its scheduled execution of a man over one kilogram of cannabis.
Singaporean Tangaraju Suppiah, 46, is set to be hanged on Wednesday after he was convicted of conspiracy to traffic 1,017.9g of cannabis, twice the amount that merits the death sentence in the city-state.
It would be Singapore's first execution in six months and the 12th since last year.
The Asian financial hub has some of the world's toughest anti-drug laws and insists the death penalty remains an effective deterrent against trafficking.
The UN's Office of the High Commissioner for Human Rights disagrees.
"The death penalty is still being used in a small number of countries, largely because of the myth that it deters crime," the OHCHR said in a statement.
"We have concerns around due process and respect for fair trial guarantees. The UN Human Rights Office calls on the authorities not to proceed with his execution," it added.
Earlier, British billionaire Richard Branson had also urged Singapore to halt Tangaraju's execution.
Branson, a member of the Geneva-based Global Commission on Drug Policy, wrote on his blog on Monday that Tangaraju was "not anywhere near" the drugs at the time of his arrest and that Singapore may be about to put an innocent man to death.
Singapore's home affairs ministry said in response on Tuesday that Tangaraju's guilt had been proven beyond a reasonable doubt.
Branson showed "disrespect for Singapore’s judges and our criminal justice system with such allegations", it added.
Cannabis has been decriminalised in many parts of the world, including in neighbouring Thailand, and rights groups have been calling for Singapore to abolish capital punishment.
Tangaraju’s family has requested a retrial.
Singapore resumed executions in March 2022 after a hiatus of more than two years.
Among those hanged was Nagaenthran K Dharmalingam, whose execution sparked global outcry, including from the United Nations and Branson, because he was deemed to have a mental disability.
Singapore invited Branson to a televised debate on the death penalty last year but the billionaire businessman declined, saying authorities should instead engage with local rights activists.