A spokesman for the Environmental Protection Department said the bill would be introduced to the Legislative Council late this year or early next year. It would provide for fines for drivers who leave their engines running and so contribute to air pollution.

The decision follows 40 meetings with interested parties, including transport trade associations, vehicle suppliers, the Heung Yee Kuk - which represents the traditional New Territories interests - and legislators. Most supported a ban.

'We will formulate a widely supported regulatory framework,' he said. 'Our plan is to introduce our legislative proposal to the Legislative Council in 2008 to 2009.'

The ban would cover all parts of Hong Kong and all vehicles except those used for emergency response and other special duties, and those with passengers alighting or boarding. The first two taxis or light buses in a queue would also be spared the ban, which would be enforced by about 200 traffic wardens, with offenders facing a HK$320 fine.

Transport trade associations having already lodged scores of objections and demanded exemptions for various types of vehicle.

At special meetings of Legco's environmental affairs panel, at least seven associations called for taxis to be exempted, while other trade groups demanded exemptions for public light buses, trucks and buses.

'No taxi drivers agree with this,' said Leung Siu-cheong, of the Taxi Operators Association.

'We are now using liquid petroleum gas for our vehicles and switching the air-conditioning on and off would do no good to the driver. We need to provide our passengers with a comfortable journey.'

Lai Ming-hung, of the Taxi and Public Light Bus Concern Group, said the exemption for queuing vehicles would be unworkable for red minibuses because they do not have assigned stops and buses for different destinations queue together.

Mr Lai said taxis would respond to the queuing rules by driving around the block instead of waiting at a stand, which would only create more congestion and air pollution.

Civic Party legislator Audrey Eu Yuet-mee, the Legco panel's chairwoman, said: 'I don't think you can exempt a specific category of vehicles. But when it comes to implementation, you will need some kind of allowances, considerations and common sense.'

Amy Ng Yuk-man, of campaign group Clear the Air, said: 'If you exempt taxis, you are going to have to exempt buses and minibuses and tour buses. The list goes on.'

Edwin Lau Che-feng, director of Friends of the Earth Hong Kong, said: 'The government should not expand the exemptions to further categories of vehicle. If there are more exemptions, the legislation will become ineffective. It needs to be across the board, otherwise motorists will question why one person has to comply with the ban while another does not.'