WHEN it comes to undercover vice operations, the police force's instructions are clear: 'It is sufficient that the prostitute behave only in a lewd manner towards the officer either directly or by suggestion', before she can be arrested. However, some officers seem to favour a more hands-on approach to tackling the sex trade. During an appeal in the High Court last month, it emerged that two officers posing as customers had allowed prostitutes to masturbate them in a bid to obtain evidence against the owner of a To Kwa Wan brothel. This was despite orders that they should have 'no physical contact' with prostitutes while on the job, a police spokesman said. Lam Chung-ming, who was convicted of keeping a vice establishment, allowing his premises to be used for prostitution and living off the earnings of prostitution, was appealing against a 10-month prison sentence and a $40,000 fine. It was turned down. The court heard that an undercover policeman was masturbated during two visits to the brothel in 1993. He returned with another officer and they were both attended to by two prostitutes before the brothel was raided. The senior inspector at the Kowloon City station who was in charge of the operation could not be contacted to comment on why his men had disregarded the force's policy. But an officer experienced in vice cases said: 'It sounds like they did it purely for sexual gratification. To be fair, in cases like this, police are caught between a rock and a hard place. 'You have to get sufficient evidence to obtain a conviction. But you get these young, red-blooded males going out to these places where they meet these women, and before they know it they have crossed the boundary. These girls work very quickly.' He added: 'I would take a hard line with my men about having sex with prostitutes because I do not want any of them coming to me in a few years and saying they have caught venereal disease or AIDS while on an undercover operation.' Both the Metropolitan Police in London and the New York Police Department warn their officers against having sex with prostitutes. A spokesman for the Metropolitan Police said their staff 'would not have intercourse with a suspect for fear of prejudicing the case. It is common sense, although we do not have specific guidelines'.