The latest concern comes just days after fears were expressed by safety organisations that the belts might be too long for children and might cause them neck injuries.

Taxi drivers and passengers fear that the stringent new law will deter larger people, including many tourists, from using taxis.

The law, which was introduced on January 1 and comes years after wearing seat belts in front seats was made compulsory, was subject to a two-week grace period, but police can now prosecute rear-seat passengers. Pregnant women and people with certain medical conditions have been told they could be exempt if they write to the Transport Commissioner. It is not clear, however, what will happen to larger people.

Ng Kwok-hung, a spokesman for the Taxi Associations Federation, said: 'I will have to make some inquiries with the Transport Department as it has not instructed us what to do in this situation - the current law has not made any provisions on such a front.

'But I personally think overweight persons who cannot be fitted with seat belts should be given exemption from the new rule. It is ridiculous their rights to hire a taxi are taken away because of this.'

Police said they were able to show discretion, but many taxi drivers are sceptical about what this will mean in practice.

David Leung Shiu-cheong, chairman of the Taxi Operators Association, said: 'If they are stopped by police, both the taxi drivers and passengers must try to reason with the officers, who will still have the power to decide whether enforcement actions should be taken.'

Ian Watson, a PR consultant and Hong Kong resident, has suffered the embarrassment of being unable to fit into his seat belt.

'I have been in many taxis since the new law came into being and invariably the seat belts do not fit me,' he said. 'What am I supposed to do? I don't see why I should receive a fine simply because the belts are too small and I am horizontally challenged.'

Frontier legislator Emily Lau Wai-hing called on the authorities to tackle the problem as soon as possible. 'If these belts are not big enough, then something must be done to address this problem,' she said. 'I don't know who is liable in these circumstances, but it must be dealt with because it's very unsatisfactory.'

Lai Kam-tong, who has driven a taxi for 10 years, warns that tourists will be put off: 'It is a great problem with pregnant women, but what about the tourists. They could get off the plane and face a fine or even jail.

'I try to remind passengers, but I cannot always see if they are right behind me and in any event it is not a good idea to look. My eyes should be on the road.'

Mr Lai said inspectors had given his taxi a stamp of approval in November when they gave it a thorough check, including its seat belts. He also pointed to another problem with the new law.

'There is a problem because many taxis do not have middle seat belts either in the front or back. What are they meant to do in that situation?

'Personally, I don't think it is a question of seat belts anyway. It is about the quality of the driver.'

Since the law came in, all new taxis have been obliged to carry the seat belts, which must match internationally agreed standards. But many of the older Toyota Crown vehicles, the standard make for Hong Kong taxis, do not have seat belts in the rear.

One Hong Kong businessman, who did not want to be named, said he had rejoiced when he found a taxi that did not have any seat belts. 'It has been a bit embarrassing really,' he said. 'You have to hold the belt across you, although in all honesty the drivers never seem to notice, or maybe they just don't care.'

The Transport Department denied the new law discriminated against larger people, pointing to the exemption rule. It said six people had applied for exemption certificates since the law came in.

'We will decide each case on its merits,' a spokeswoman said.

Ms Lau, however, said there was an alternative solution for some of the larger passengers. 'A diet would certainly do the passenger a lot of good,' she said.