A paedophile who committed 18 offences of abuse over 20 years has been jailed for five years after abusing his own godson, a Hong Kong news report said Thursday.

The 50-year-old, who was both tutor and godfather to the 13-year-old boy, appeared in court for sentencing Wednesday after earlier pleading guilty to four charges of committing indecent acts with a child under 16.

The court heard he had molested the boy on occasions between November 2007 and January 2008 after showing him pornographic films at the boy's home.

The offences came to light when the boy wrote about them in a letter to his father.

The accused was a diagnosed paedophile who had committed 18 similar offences over the past 20 years while he was running a tutorial school, said the report in the South China Morning Post.

His lawyer claimed he had been deprived of love as a child and had not treated the boy merely as a sex partner but like a partner, sharing his feelings and thoughts with him.

But Judge Judianna Barnes Wai-ling rejected the claim calling him a menace to society" who had taken advantage of a child.