Toyota Motor and Honda Motor are leasing a handful of the cars to the Japanese government and several public establishments in the United States in an experimental programme that marks the biggest step yet towards the mass marketing of fuel cell vehicles (FCVs).

The ultimate "green car", FCVs could be part of the solution to smog, global warming and other ecological problems that conventional cars help cause.

The technology, which was first used during the Apollo moon project in the 1960s, mixes hydrogen fuel and oxygen from air using an electrochemical process to produce the electricity that powers the car.

Far from harming the environment, its only by-products are heat and water - water so pure the Apollo astronauts drank it.

Many of the world's biggest carmakers want to make FCVs available to the average consumer. If all goes as planned, FCVs may begin replacing gasoline-powered cars in the next decade.

However, carmakers still haven't figured out how to make FCVs at an affordable price, or how to build enough fuelling stations - and rapidly enough - to make them practical.

The high costs of research would force car firms to charge anything from $1 million to $2 million for every FCV initially.

"There are still many challenges left for full-blown commercialisation," Honda President Hiroyuki Yoshino said at a handover ceremony at the prime minister's office.

Leasing the first FCVs won't be cheap, either.

Three Japanese ministries and the Cabinet Office will fork out a hefty $9,800 a month to rent Toyota's five-seater "FCHV". Honda's four-seater "FCX" will cost $6,500 a month in Japan.

BEST ALTERNATIVE

Still, FCVs are considered the most promising alternative to today's gasoline-fuelled cars.

Hydrogen is the most abundant element in the universe. Oil supplies, on the other hand, are finite, and global oil production could peak by 2020, according to a U.S. government report.

That means even gasoline-electric hybrid cars, the most fuel-efficient cars around now, will lose their power source one day.

Unlike pure electric cars, FCVs don't need to be recharged. They can run for at least 300 kilometres (186 miles) before refuelling, at a speed of about 150 km an hour (93 mph).

With automotive vehicles believed to be responsible for a third of the world's greenhouse gas emissions, which lead to global warming, governments have recognised the urgent need to encourage cleaner cars.

"When I took office last year, I promised that in three years we would replace all cars used by the government with low-emission vehicles, even if it costs a little more," Japanese Prime Minister Junichiro Koizumi said at the ceremony.

"It's important that we continue to develop green cars."

The United States is doing its part, too.

Although the country pulled out of an international treaty to reduce greenhouse gas emissions, its biggest auto market, California, has been aggressively leading the nation's drive for stricter standards for emissions and fuel efficiency.

California, which has the unique right to set its own emissions regulations, is calling for all cars sold in the state to have near-zero emissions by 2009, which could set a precedent for federal legislation.

The state is leading by example. The FCVs launched today will be leased to two California universities by Toyota and the city of Los Angeles by Honda.

The United States is also keen to reduce its dependence on oil from the Middle East, and fuel cell technology is one answer.

BARRIERS, HAZARDS

In addition to price, the question of refuelling stations could be the biggest barrier to winning a mass market.