The District Court heard Cheng, 23, was already serving a two-year sentence for being a pimp. He was kicked out of the force when convicted of the offence last October.

Yesterday Judge Britton told him: ''You are a disgrace both as a man and to the uniform you wore for four years.'' Cheng was suspended from the force in 1992 for allegedly having unlawful sexual intercourse with an underage girl. After an inquiry he was charged with living on immoral earnings.

His accomplice in the blackmailing, Cheung Chin-ting, 34, was also sentenced to 21/2 years in jail when found guilty of theft, blackmail and claiming membership of a triad.

The pair went to the Ho Fat Metals and Paints Shop in Yau Ma Tei at 2 pm on March 29 last year, its owner Chow Tuk-kwong had told the court.

Cheung claimed to be a member of the 14K triad society and demanded $8,000 protection money from him, he said.

Cheng paced up and down menacingly, so he handed over $6,000 and told them to come back for the rest. But Mr Chow tipped off the police who arrested the men when they returned three days later. Both men denied the charges.

Cheung told the court he went to the shop to offer Mr Chow $2,000 a month for electricity and water supplies for a tea stall he proposed to set up nearby.

Cheng claimed he never entered the shop and was only there to look at the site for the stall.

But Judge Britton said: ''Mr Chow is a decent, salt-of-the-earth, spirited man who gave an honest account of what happened.

''His wife [Yeung Suet] was a timid woman who, once she had settled down, gave a lucid, calm account.

''They did not strike me as being the sort of persons who would invent this story to stop a tea stall opening in the lane.

''The demand for $8,000 was wholly unwarranted and the mention of 14K completely designed as a menace.'' Tom Cheng, defending Cheng, pleaded for a short sentence because police officers were often roughly treated in prison.

John Haynes, defending Cheung, said in mitigation that his client resorted to crime because he was suffering badly in business.

Judge Britton said nine months of Cheng's sentence would run concurrently with his present jail term.

Afterwards Mr Haynes said his client was considering suing the police for alleged brutality.

A statement Cheung made at Tsim Sha Tsui police station was ruled inadmissible after he claimed officers beat him with an iron bar and broke his foot.