One agent's sales manager called the prices 'crazy'. A director of another said a Chinese International School debenture now cost more than memberships for clubs such as the Aberdeen Marina Club.

Days after the Sunday Morning Post reported a couple had paid HK$1 million for a debenture to secure their daughter a place at Chinese International School, one parent hoping to win her child a place there was quoted HK$3 million for a corporate debenture. The same agency quoted her HK$1.4 million in September.

Another of the five agencies believed to be selling debentures quoted the woman HK$2.1 million, a 20 per cent increase on the HK$1.75 million it quoted a month earlier.

Fuelled by demand for places at international schools and long waiting lists, debenture prices for Canadian International School have also soared on the second-hand market.

Chinese International School corporate debentures have a face value of HK$600,000 and Canadian International School quotes HK$300,000 for an individual debenture.

The Post asked the two schools to disclose the value of the most recent second-hand debenture sales each had approved.

Chinese International did not reply. A spokesman for Canadian International said: 'The most recent [individual debenture] sold for HK$400,000.'

The mother seeking a Chinese International School debenture said she had been trying to arrange a place for her daughter at an international school for more than a year after her employers agreed to buy a debenture for the girl. The company had gained approval from Chinese International School in the autumn to buy a corporate debenture.

'After we got the approval, we started calling around but there was just nothing on the market,' she said.

She began inquiring again in March. Five weeks ago, she consulted China Dragon Membership Services, and went back to the agency for a fresh quote last week.

'When I looked on May 10 it was HK$1.75 million, and when I looked again it was HK$2.1 million,' she said. 'I was flabbergasted that it should go up by HK$350,000 in one month.'

On Friday, she was quoted HK$3 million by Elite Membership Services.

'We are appalled, not only at the alarming prices, but the alarming rates at which they are increasing,' she said. 'These prices are non-transparent, unverifiable, and clearly spiralling out of control. I feel strongly that the schools should address the issue.'

A parent who has a child at Chinese International said: 'People will pay any price. The government will have to do something before long.'

Yeung Sum, who is deputy chairman of the Legislative Council's education panel, said: 'I urge the Education and Manpower Bureau to try to do something. It will affect the image of Hong Kong and place an unreasonable burden on parents.'

A bureau spokeswoman said it would be inappropriate for the government to micromanage international schools in the private sector.