Asked about the de facto red-light district, police said prostitution was legal in Hong Kong, but that soliciting, living off the earnings of a prostitute or controlling a woman for the purpose of prostitution was not.

Renting a flat or apartment to a prostitute does not make it a vice establishment in law. It is only classed as such if more then one prostitute is working there.

The Fuji Building may be full of prostitutes, but so long as each individual flat can be shown to be a separate unit from all the rest - that it has its own water connection, electricity meter and no one else can access it - it is above board.

'The majority of these prostitutes are Hong Kong [identity] card holders who have a legal right to work in Hong Kong,' a police spokesman said. 'The problem is that sometimes people let two-way permit holders [from the mainland] and illegal immigrants live in these buildings.

'It's for this reason that we have raided the Fuji Building and others like it in the past. Not because they are breaking prostitution laws, but because they are breaking immigration laws, as two-way permit holders, or illegal immigrants are living there.

'Foreign prostitutes that are soliciting in the street or in bars are also working here on tourist visas, so when we arrest them it's for breaching the condition of their immigration status, not for prostitution.'

Police said those renting the small apartments to the prostitutes in places such as the Fuji Building were also doing nothing illegal. Many of the women live there: their place of work is also their home.

'There's a possibility that the landlords are taking a cut from what the girls make, which would mean they are living off the earnings of prostitutes and so can be arrested, but it's very difficult to prove that the landlord's intent is to take a cut from these girls' takings,' the police spokesman said.

'It has been our experience that this does not happen.

'In most cases the girls are not making enough for anyone to get a cut of their takings anyway. They'd pay their rent and that'd be it.' Police also said a landlord might unknowingly rent the premises to someone who sublet it to a prostitute.

Websites such as Sex 141 advertise premises such as the Fuji Building and the services of the prostitutes in them. Hong Kong police cannot shut down these sites, since they are operated from the mainland.

Lawyer Michael Vidler, who has experience of the issue, having handled immigration cases, believes it would be best to make prostitution legal across the board.

'This is something that's not going to go away,' Vidler said. 'It would be better if it was regulated so that there were regular health checks for sex workers, that they could pay their taxes and it would take it out of the hands of organised crime.

'In my view it's freedom of labour. If women voluntarily want to do this then they are perfectly entitled to.'

Documents obtained from the Land Registry show that the Fuji Building was initially bought by Chang Pao-ching in 1971, but that today a variety of landlords own apartments, on different floors.

The premises in the Land Registry documentation for the building are referred to as 'office' or 'shop', not 'flat' or 'apartment'.

Of the four floors not used for prostitution, one comprises flats, and the other is marked as the headquarters of the Hong Kong Federation of Writers.

When the Sunday Morning Post visited the building, the general protocol for customers seemed to be to take the lift to the top floor, then walk down the stairs, taking a look at prostitutes on different floors after knocking on their doors.

Men were milling around.

The price for sex ranged from HK$400 to HK$500.

Closed-circuit television cameras were in operation in lifts and in the passageways of each floor.

Similar buildings can be found in Central, Tsim Sha Tsui and elsewhere in Kowloon. Each functions as a self-contained red-light district.

'There are many buildings in Hong Kong where sex workers are concentrated together: it's very common,' said Ng Nga-shan, project co-ordinator for Zi Teng, a non-governmental organisation formed to protect prostitutes' rights in Hong Kong. 'They will not get any complaints from neighbours, unlike in a residential building.'

This does not guarantee the women's safety, however.

'It's down to the landlord to decide whether there will be CCTV installed or that there is a panic button in their apartment if something goes wrong,' Ng said. 'They have to ask the landlord to install these things, and if he says 'no' it's a big safety problem for them.'