Twenty-one alleged Malaysian 'drug mules' are facing the death penalty in Thailand.

The Thai police said investigation is under progress after the Malaysians were detained with large quantities of methamphetamine and heroin. If convicted, the drug carriers could be sentenced for death under the country's Narcotics Act for possession and sale of category 1 substances.

"The suspects can face the death penalty following the large seizure of methamphetamine ('ice') and heroin. The police have obtained strong evidence against them," Thai police said, according to The Star.

"However, despite the possibility of facing the maximum sentence under the stipulated charge, Thai courts seldom hand down the death penalty and prefer the long-term jail sentence, instead," he continued.

On March 23 and 24, the Thai Railway police arrested the Malaysians, in two groups of 15 and six men from at four different train stations and in a passenger van. They were travelling in a train which was in the route to Butterworth from Bangkok.

The police seized 226kg of methamphetamine and 8kg of heroin from them as they travelled on one of the most notorious drug trafficking routes in Southeast Asia.

Thai police said the "drug haul" was one of the largest in recent times. When asked about the value of the contraband, they said that this could fetch about RM400mil in Europe.

Thai police also said that based on information obtained, there was a link between the Malaysian suspects and a major drug trafficker whose nationality he has declined to mention.

In a previous interview with Bernama, Police Col Puttidej Bunkrapue from the Thai Railway Police had said that the investigations revealed that the drug smuggling attempt by the Malaysian suspects was masterminded by three men.