At present, the Forests and Countryside Ordinance makes it an offence to damage any tree on government land. But critics say it focuses mainly on 27 rare species of trees, while many other old and valuable trees do not receive special care from authorities.

'The existing law might not be able to save a tree grown by Dr Sun Yat-sen, or one aged over a century,' Ms Choy said.

The proposed bill, which applies only to trees on government land, will protect trees aged 100 years or more; those with a trunk diameter over 0.8 metres, height over 15 metres or girth over 2.5 metres; and those with historical, cultural or memorial significance or scientific research value.

The bill will be submitted to the legislature in the upcoming session if consent is secured from Chief Executive Tung Chee-hwa. Under Article 74 of the Basic Law, a private bill that affects government policy and public expenditure requires such consent. The bill will introduce a new provision to prevent damage or vandalism to such valuable trees by preventing any change of land use of their sites.

In addition, the bill will protect not just the tree itself but the area five metres around it, vertically and horizontally.

Breaking the ordinance would be punishable with a fine of up to $100,000 and two years in jail.

'Only under exceptional cases, involving public safety and public interests, can these trees be cut or removed,' Ms Choy said.

It is the second attempt to legislate tree protection in a year. Last year, Jim Chi-yung of the Geography Department of Hong Kong University submitted a draft ordinance on urban tree protection for the government's consideration. It is still being scrutinised.

Professor Jim said: 'Simply protecting the old trees won't make our cities greener, as we also have to protect the green belt in the periphery of the city and in the parks. We also need a clear set of rules for tree-felling.'

A spokeswoman for the Environment, Works and Transport Bureau, which co-ordinated Hong Kong's greening effort, yesterday said sufficient protection for trees has already been legislated, including measures under the Crimes Ordinance.

'We welcome the draft bills proposed by Professor Jim and Ms Choy on tree preservation and greening,' she said. 'But there is already a set of comprehensive legislative and administrative measures in place for such purposes.'

She said the government has registered 950 old and valuable trees across the territory. It will issue a technical circular, setting out the requirement for protection and maintenance of the registered trees, to various departments, she said.

The government will also review tree-felling procedures, and explore the feasibility of tree surveys on sites before land is sold, to identify trees for preservation and protection, she said.