After all, it was only four years ago, in February 2001, that the environment secretary made a similar promise to legislators, telling them: 'We have given very explicit guidelines that all government vehicles must implement idling-engine control.' Those guidelines were quickly forgotten. Who is to say that new ones will be taken any more seriously?

Mr Tsang said he hoped private car drivers would follow the example of government drivers. But if the government's own employees flout the guidelines, what hope is there that others will switch off their engines? The government's policy of using persuasion rather than legislation on this issue has been a demonstrable failure - one that shames Hong Kong. In my view, Mr Tsang did not go far enough.

When the Legislative Council resumes sitting, I will ask for this matter to be re-examined by the government, and for its advisory approach to be abandoned in favour of fines and prosecutions.

For the past five years, we in the Liberal Party have been pressing for legislation against idling. When I raised the issue in Legco in 2001, I was told that the government favoured the advisory approach because legislation would be difficult to enforce and motorists would simply drive around, producing even more emissions, rather than sitting in a car with no air conditioning.

I did not, and I do not, accept those arguments. An advisory approach is only any good in the short term. In the end, legislation with limited exemptions is the only long-term solution. As the law against littering has shown, Hong Kong people are law-abiding. Once a law has been added to the statute book, the vast majority of people will respect it.

The notion that drivers will cruise around aimlessly rather than pull over and switch off their engines is fanciful. Anyone who has tried negotiating the city's congested roads will know that a quick drive around the block can easily turn into an hour-long detour.

What the government means when it employs phrases like 'counterproductive' and 'difficult to enforce' is that it does not have the political will to tackle a growing menace.

The volume of letters on the subject in the South China Morning Post in the past two months shows that the government is out of touch if it does not think this is an issue people consider a priority.

Air pollution is one of the most serious problems facing Hong Kong. It is true that the majority of the pollutants drift down from industrialised southern China, and we must take steps quickly to reduce those pollutants.

But how can we strive for progress in our co-operation with the Guangdong authorities when we are not prepared to deal with the far less complex and less politically sensitive problems in our own backyard?

We need a law against idling vehicles and we need it as a matter of urgency. The time for talk is over. It is time for the force of law to be brought to bear on polluting motorists.

James Tien Pei-chun is chairman of the Liberal Party