A new underclass could balloon, with hundreds of thousands of unskilled, uneducated and poor growing increasingly discontented as the SAR's economic star dims.

Academics warn this could be the disturbing reality for Hong Kong by 2004, unless the Government quickly assesses the number of mainlanders expected and starts work on the infrastructure required.

'If we take the maximum impact scenario, the figures being bandied around are [a total of] 400,000 over and above the [current one-way permit quota of] 150 a day,' said Professor Peter Hills, director of the University of Hong Kong's Centre of Urban Planning and Environmental Management.

'So if we're looking at, say, a four-year period, it's up to 600,000 people. No city in the world, at this stage of development, faces this kind of population pressure,' Professor Hills said.

Many of the arrivals would be middle-aged and almost all would seek low-rent private housing or join waiting lists for public housing. They would also be low-skilled workers, going head-to-head with locals for a shrinking pool of labouring jobs.

An underclass with festering social problems could result, because joblessness among the poor, low-skilled group would far outstrip the average figure for the total population.

'There's going to be a growing group of young school-leavers who are finding it very hard to find employment,' Professor Hills said.

'The magnitude of the population problem is so enormous it leaves you scratching your head as to where they can be put.

'Basically, we're talking about a new town - certainly as big as Sha Tin was a few years ago - within a space of about four years.' Government planners will be hard pressed to accurately calculate the numbers due to arrive following the ruling and predict their ages, education and job levels and where they intended to live, experts say.

But fast, accurate predictions are needed if Hong Kong is to survive the wave.

'The environmental system - the air, water, waste disposal - are at or over capacity now. We're pushing up against the limits with 6.7 million people,' Professor Hills said.

'And how's it going to affect tourism? Hong Kong is a crowded place, but there's a difference between a crowded, thriving, dynamic city and one that's overloaded.' Friends of the Earth director Plato Yip Kwong-to said Hong Kong was already over-populated and even another 100,000 people would create pressure on the strained environment.

'We respect the court's decision, but we need to study the consequences and how we will deal with the population pressure,' he said.

Baptist University associate professor of sociology Dr Siu Yat-ming said many Hong Kong residents were originally from Guangdong, so their offspring would be from the same province.

'If you study the profile of the new arrivals from Guangdong, most of them will be from rural areas, looking for low-skilled and low-education jobs,' Dr Siu said.

Past schemes aiming to attract highly educated professionals from other provinces to Hong Kong had fizzled because they were unable to bring their families, he said.

A sex imbalance had also been building because, although half of the daily one-way permit quota went to children under 21, mainland wives made up the bulk of the remainder of arrivals.

There were already large numbers of unskilled women vying for work such as cleaning jobs.

University of Hong Kong assistant professor of urban design Ho Chi-wing said districts that would come under heavy demand for extra housing and schools had to be immediately pinpointed.

'Some people talk about building a village of public schools all in one place and sharing facilities, like a gym, pool and so on, and students from all over the territory would be bussed there.

'We need to plan right now - we need the information as soon as possible.'