"Singapore maintains that capital punishment is a criminal justice issue," a foreign ministry spokesman said.

"It is the sovereign right of every country to decide whether or not to include capital punishment within its criminal justice system."

Philip Alston, from the UN Human Rights Commission's watchdog on extrajudicial, summary or arbitrary executions, urged Singapore to stop the planned hanging of Hguyen, who was convicted of smuggling heroin.

He said the problem in the case lay in mandatory sentencing rules that required the death penalty to be imposed in trafficking cases.

"Making such a penalty mandatory -- thereby eliminating the discretion of the court -- makes it impossible to take into account mitigating or extenuating circumstances and eliminates any individual determination of an appropriate sentence in a particular case," Alston said.

"The adoption of such a black-and-white approach is entirely inappropriate where the life of the accused is at stake."

The Singapore foreign ministry accused Alston of "grossly misrepresenting the facts".

"We regret that Mr Alston has attempted to mislead the public. In doing so, he diminishes the credibility of his office.

"Mr Nguyen was tried and convicted in an entirely open, fair and transparent manner, according to due process of law, as has been acknowledged by the Australian government. Therefore this case does not fall within his mandate.

"Mr Alston has on several previous occasions attempted to exceed the limits of his office in criticising judicial executions in Singapore.

"We have previously protested Mr Alston's abuse of his office and will continue to do so as necessary."

The Australian government has mounted a diplomatic campaign to save the life of Nguyen, a 25-year-old ethnic Vietnamese arrested in Singapore in 2002 with 400 grams (14 ounces) of heroin while in transit between Cambodia and Australia.

But it has so far been to no avail.

The death sentence is mandatory in Singapore for trafficking more than 15 grams of heroin, 30 grams of cocaine or 500 grams of cannabis, as well as for other crimes such as murder, treason, kidnapping and certain firearms offences.

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