''Chek Lap Kok can cope with up to 80 million passengers, and I very much doubt it will fill up that quickly.'' A private sector consortium had asked the Government to keep part of the runway at Kai Tak, set to close in 1997, for use as a commuter airport.

But Mr Siu said the Economic Services Branch had rejected the idea because it would cause air traffic control problems.

''Kai Tak is completely unsuitable for use as a commuter airport,'' he said.

''In London the direction of the runway of the city airport is identical with the international airports. But Kai Tak and Chek Lap Kok are diagonal to one another, and that causes problems with aircraft handling.'' Another possibility was the airstrip at Sek Kong, but Mr Siu said he did not think that idea had been proposed. ''There has been no study,'' he said.

Mr Siu said commuter aircraft were small jets carrying around 50 passengers bound for cities up to 300 kilometres away, such as Guangzhou and Xiamen.

He said small commuter planes could use Chek Lap Kok while the airport still had spare flight time. ''I see no objection to smaller aircraft using it,'' he said.

''When demand builds up; when it reaches capacity, that is a separate matter, but that will be well into the century - 2040. We have got a lot of time until then.'' But Anthony Charter, managing director at Hong Kong Air Cargo Terminals, said it would be better to keep everything at Chek Lap Kok.

''Why not concentrate everything there?'' If both runways were going to reach capacity, he said, the Government should build a third at Chek Lap Kok as long as there were no air traffic control problems.

Despite Mr Siu's denial of any firm plans to build a commuter airport at Kai Tak, the proposal is expected to be considered again during a planning and design consultancy to be awarded later this year.

Several consultants are already working on studies covering Kai Tak but these are considered to be highly secret.

Transport experts said a commuter airport would cost a fraction of the $60 billion it is taking to build Chek Lap Kok airport.

Adapting the end of Kai Tak's runway to a commuter airfield would cost less than $30 million yet leave 70 to 80 per cent of the site available for property development, according to estimates produced by Robert Fieldman, a construction expert working forSaab Aircraft.

Based upon the construction cost of similar airports, the only facilities which need to be fixed are a passenger terminal and hangar, which could be supplied as portable buildings, and some fencing.

Only 1,500 metres of the existing runway would be needed. Because the aircraft were so quiet they would have little impact on the community.

Kai Tak's continued use as an airport would also be welcomed by business people used to its convenience.

The proposal has been backed by Sir Piers Jacobs, the former financial secretary, and the Kadoorie group.

Martin Craigs, president of the Asia/Pacific region of Saab Aircraft also supported the idea, saying Kai Tak would be more convenient than Chek Lap Kok for people flying within a 1,500-kilometre radius of Hong Kong.