Two Singaporean traffickers were spared capital drug charges after the High Court was not convinced they knew they had 2lb (907g) of a granular substance which contained 21.58g of diamorphine, or pure heroin, instead of 1lb of the substance as they claimed.

Justice Choo Han Teck on Wednesday amended the charge and convicted them of jointly trafficking in 10.17g of diamorphine instead, based on the evidence from a six-day hearing in February.

Ng Peng Chong, 58, a pimp, and Cheng Pueh Kuang, 59, a former cab driver, were arrested by officers from the Central Narcotics Bureau as they were parking their car in Lorong 33 Geylang on May 16, 2014. They were coming back from Woodlands, where they had collected the heroin from a supplier. They surrendered to the officers and admitted they had heroin in the car, when asked.

A search of the car yielded a black bundle which was found to contain two packets of a brown granular substance weighing about 2lb in total. The substance was found to contain 21.58g of pure heroin.

Deputy Public Prosecutor Isaac Tan said both men had jointly ordered and collected the items for the purpose of trafficking.

The accused did not dispute the fact that they had ordered and received the heroin from their supplier earlier that day.

But defending lawyers, Mr Cheong Aik Chye appearing for Ng, and Mr Peter Cuthbert Low for Cheng, argued that the duo had ordered only 1lb of the substance.

Although they were found with 2lb, they thought they had only 1lb and had not intended to be in possession of 2lb of heroin.

As evidence, they pointed to the text messages between the accused and the supplier, which suggested they had ordered only 1lb of heroin. In addition, their past modus operandi showed they used to order no more than 1lb or half a pound at any time.

The lawyers also argued that the men intended to consume about 6g of the drug themselves.

In judgment grounds issued on Wednesday, Justice Choo found their evidence to be consistent and credible throughout and it was "likely" they ordered 1lb to be delivered on May 16, 2014.

"Despite the lack of clarity about when the heroin was ordered, they have consistently maintained that they had only ordered one pound of heroin, from the point of their arrest to the recording of their long statements, and to their oral testimonies at trial," said Justice Choo.

"Their version of events is corroborated by their text message history, which reflects a general pattern of ordering only one or half a pound of heroin," he added.

The judge noted, among other things, that the prosecution had not contested their evidence that their orders for heroin were reflected accurately in the text messages.

Justice Choo said it would be "artificial and harsh" to find that they had become aware of the additional pound of heroin during the drive from Woodlands, where they had collected the drug, to their home in Geylang.

He amended the charge from joint trafficking of 21.58g of diamorphine analysed from the 2lb of heroin in its gross form in the two packets, to joint trafficking of 10.17g of diamorphine contained in one of the two packets, which averted the capital charges.

Both were found guilty and convicted of the lesser charge and are due to appear in court again later this month for sentencing.