The term - used in the zoning rules of the Town Planning Ordinance - had previously also included construction sites for which building plans had been approved by the authorities.

Under the old rules, all 'existing buildings' were exempt from new town planning restrictions, such as construction height limits or population density.

The change means that, in theory, the Town Planning Board can now call a halt to an approved property development when it re-zones an area of the city.

The association, whose membership includes Hong Kong's biggest property developers, opposed the change, but in February the board refused to halt the changes.

Its rejection summary said the amendment clarified existing policies.

'A proposed building should not be regarded as an existing use as the building is not yet in existence,' it said.

Louis Loong Hon-bin, secretary general of the developers' group, said the change 'threatens private ownership and the rule of law'.

'The impact of this change is not going to be confined to just real estate developers. It will affect every property owner in Hong Kong and weaken investment confidence,' Mr Loong said.

'Before the change, the right to develop a site was a statutory right. Now it is at the mercy of the government's administrative measures.

'We [are willing to] launch a judicial review against the board's decision. But it is not our preferred option. We want to try the Executive Council first.'

Mr Loong was confident Mr Tsang would see their viewpoint.

A spokesman for the Town Planning Board said the amendment would not affect a company's right to develop its sites.

He said development proposals that have been approved would not be affected by the change unless major amendments were made to the approved building plans.