The bureau told of the change in response to e-mail queries on how to improve access to international schools for expatriates.

At present, international schools using government land or premises can draw up to half of their students from the local population. But the rules do not apply to schools with their own properties, some of which are taking in up to 80 per cent local students.

A bureau spokeswoman said the new policy was aimed at 'ensuring that the increase in international school places can catch up with increasing demand from non-local families'.

But legislator Emily Lau Wai-hing, vice-chairwoman of a government body set up to encourage business investment, said such a cosmetic change would mean little as it did not apply to existing schools.

'It only applies for future schools,' she said. 'The government does not appear to be comfortable about imposing controls.'

Affluent Hong Kong families and senior government officials tend to prefer sending their children to English-speaking schools in the city or overseas amid concern about the erosion of English standards since the handover.

The sight of parents speaking to toddlers in English in shopping malls and other public areas has also become common as families strive to raise the language skills of locally born children to a level where they can pass proficiency tests at international schools.

Education sector legislator Cheung Man-kwong said at a Legislative Council meeting in March that foreign students had become the 'minority' in some schools. He urged the government to take action so that non-Chinese families have access to the education that suits them.

According to official figures from the last school year, the local pupil population has reached 60 to 80 per cent in several international schools.

The government cannot intervene in these cases as it does not subsidise them, Secretary for Education Michael Suen Ming-yeung said.

Only about a fifth of some 50 international schools in Hong Kong are operated on land or premises subsidised by the government. But officials say only about 13 per cent of students going to international schools are from Hong Kong.

Schools that have recently obtained land in the city for their operations - such as Harrow International School in Tuen Mun - are sticking to the 50 per cent policy.

So are all the other schools that have received approval in the past two years to use vacant government premises as temporary campuses to expand their capacity, according to the Education Bureau. These include Singapore International School and Hong Kong Japanese School.

The bureau spokeswoman said that the new rule would only apply to schools that win bids in the next allocation exercise, but it is unclear when that will start.

Lau, who is vice-chairwoman of the Business Facilitation Advisory Committee, set up by the financial secretary in 2006, said the lack of international school places was discussed at a November 28 meeting at which members expressed concerns.

School officials said it was difficult to gauge whether or not locals predominated. 'It is not possible to distinguish between local students and foreign students because many local students have all kinds of different passports - like British, American, Canadian, Australian passports. We don't even keep statistics on this,' said Trevor Wilcox of the German Swiss International School.

Some educators say strong demand from local students for international school places has been fuelled by a perception that the city's education system cannot keep up with overseas standards.

'Parents have told me this is a huge issue,' said teacher Ken Buchanan of Think International School. 'They were worried about children doing so much and working so hard but also [immersed] in a stressful environment so that their passion to learn gradually erodes.'

More than 80 per cent of Think International's students are locals. The trend is set to continue, according to Buchanan, as more parents opt for the International Baccalaureate curriculum, put off by frequent changes to the local system.

The English Schools Foundation - which is in dispute with the government over the future of its subsidies and its rules of operation - is not subject to any restrictions on the proportion of local students, which comprise less than 50 per cent of its total.