Ken Livingstone's officials have threatened to abolish a loophole permitting "green" cars to escape the London mayor's £5-a-day congestion charge amid signs that thousands of the capital's motorists are developing a new-found concern for the planet.
A clause in the charging scheme, which begins on February 17, allows vehicles using alternative fuels to enter central London without paying.
The get-out clause has prompted a flurry of motorists and corporate fleets to convert their vehicles to liquefied petroleum gas (LPG). The energy saving trust, a government-funded body which offers grants for green vehicles, has seen weekly applications soar by 80% since November.
A spokesman for Mr Livingstone's Transport for London authority said the mayor's advisers were "keeping a pretty damn close eye" on the trend. He pointed out that the congestion charge was supposed to stop traffic jams in the capital, rather than merely reducing pollution.
"If the exemption gets out of hand, we will act," he said. "That could mean tinkering with the charge.
"In the most extreme case, if it turns out to totally defeat the point of the charge by encouraging tens of thousands of green vehicles to drive into London, we would terminate the exemption."
About 25,000 vehicles on Britain's roads use LPG. Most cars can be converted at a cost of about £1,500, of which the energy saving trust will refund 60%. Although rising rapidly, the number of conversions is still modest.
A spokeswoman for the trust said applications had risen from 30 to 55 a week, with 2,353 since September: "For the first time ever, there's a real monetary advantage in switching to alternative fuels. Word-of-mouth reports suggest that fleets are really looking seriously at changing over."
Marks & Spencer and Tesco are among the big fleet users. A spokesman for Vauxhall, which makes LPG versions of its Astra, Vectra and Omega cars, said: "There's certainly been an increase in interest over the last month or so, although it's too early to say if it will translate into orders."
Alternatives to LPG include natural gas and electric power. But LPG is by far the most popular, as most vehicles manufactured since 1997 can be adapted.
Recent corporate converts include Bunzl, which uses 150 vans to restock vending machines in central London.
Bhasker Dave, fleet manager for Bunzl, said the congestion charge could cost the company up to £75,000 a year. He has opted to go green instead: "We're in the process of converting our entire fleet to LPG."
He said if the company paid Mr Livingstone's charge it would have to employ an extra member of staff to handle all the daily transactions: "It creates a lot of extra administrative work. For companies like us, it's certainly a hindrance."
Experts say the main downside to alternative fuels is a lack of petrol stations which stock LPG. Just over 1,000 fuel stations have it in Britain, although the cost of land in London means many fuel stations in the capital have insufficient space to stock it.
Environmentalists have welcomed the rebate for green vehicles. But Steve Hounsham, a spokesman for Transport 2000, criticised the mayor for casting doubt on the exemption.
· Gas guzzlers
For most motorists looking for "clean" fuel, the most practical option is liquefied petroleum gas, a mixture of propane and butane. Many new vehicles are available in LPG versions, and most cars built since1997 can be converted for about £1,500. LPG emits up to 63% less carbon monoxide than petrol, with reductions in nitrogen oxides and hydrocarbons.
The government has cut tax on LPG to encourage its use. Duty on it is 4.5p a litre, compared with 50p for traditional fuel. To qualify for exemption from London's congestion charge, LPG vehicles must fulfil the highest "clean" criteria - band four of the PowerShift register (on PowerShift's website: www.powershift.org.uk).
Natural gas consists mainly of methane. Vehicles need to carry a high volume of compressed gas, and it is more suitable for lorries and buses.
Natural gas costs as little as 6p a mile, compared with 10p for standard petrol. Cars can be converted from about £2,000. Many natural gas vehicles have "bi-fuel" systems, which allow drivers to switch from gas to petrol.
Electricity tops the clean league, producing no tailpipe emissions and making very little noise. About 1,800 milk floats in Britain run on electricity, but vehicles usually need to be recharged. Most electric cars can be recharged from a standard socket, but charging takes up to seven hours.
Hybrid cars, using electricity and petrol (such as the Honda Insight below) are a more practical option. These use electricity at lower speeds but switch to fossil fuels outside towns, at faster speeds or when batteries are low.
