A supergrass who brought to justice one of Hong Kong's top drug barons had his sentence slashed by seven years yesterday after receiving a letter of support from a High Court judge. Szeto Tsz-kwong, 26, lives in constant fear of revenge attacks after betraying a gang boss responsible for flooding the territory with heroin. His testimony led to Tam Kai-ching, 54, being jailed for 30 years for bringing drugs worth $60 million in from China. Mr Justice Edward Bewley, who sentenced Tam, took the rare step of writing a letter to the Court of Appeal stressing the importance of Szeto's evidence. He said his testimony was 'very persuasive' and was responsible for Tam's conviction. Szeto was jailed for 16 years in March last year after pleading guilty to trafficking in more than 10 kilograms of heroin. His jail term was cut to nine years yesterday when he was classed as a supergrass even though he had only given evidence against one person. At the time Szeto received his original sentence, Tam was still on the run. Graeme Hagon, defending, said Szeto did everything he could to help the police catch the drug baron and put him behind bars. He provided information that enabled Narcotics Bureau officers to arrest Tam as he tried to leave Hong Kong last July. Szeto is serving his sentence in conditions of high security to protect him from other prisoners. His wife was threatened that she would be harmed if he gave evidence against Tam and the police have helped her move to a secret address. Mr Hagon said Tam's high rank in the drug world posed a real risk of Szeto being attacked. 'He is a man who presumably has the money, access and capability of exacting revenge on Mr Szeto or his family,' Mr Hagon told the court. Szeto should be treated as a supergrass even though he had only given evidence against one person. Mr Justice Simon Mayo said Tam's conviction was 'a matter of great importance'. Szeto's actions called for a substantial discount in the sentence, he said. Szeto, a labourer, had led a decent hard-working life until he agreed to become a trafficker under pressure from Tam. His family had financial problems and he could not resist the $20,000 a month he was offered for joining the gang. Szeto was arrested when police raided a building and found him with 30 blocks of heroin.