But some fear backers of growth firms on London's junior exchange, the Alternative Investment Market (AIM), could lose their shirts.

"It has all the makings for a bubble," said Keith Woolcock, director at Westhall Capital, a London-based stockbroker. "It could take a while to pop because we are in the very early stage."

Like Google Inc. and Microsoft, a handful of tiny companies could grow into profitable global players, he added.

"There's going to be a lot of investors who get burnt, there's going to be a lot of investors who are going to make an extremely large amount of money," Woolcock said.

Spotting a winner is hard in a sector which sees profit in anything from pig manure and trapped mine gas to the oil of the tropical jatropha tree and the humble plastic recycling box.

"There are a lot of pitfalls," said Craig Pennington, global energy portfolio manager at Schroders, the asset manager. "There are a lot of losing money opportunities. I would definitely urge caution."

Even companies with a good idea have no guarantee of success of beating established companie with big research budgets.

"You have to have some degree of confidence that you're backing the VHS of alternative energy and you're not backing the Betamax," Pennington said, referring to Sony's defeat in the 1980s battle for the home video market.

GREEN REVOLUTION

Dozens of "green" companies seeking capital have joined the London Stock Exchange's AIM, attracted by its looser regulations, lower fees and good access to capital.

Many have seen their shares rise sharply, and often fall again, as the industry attracts greater attention.

One of the biggest areas of investment in the rush to join the green revolution is renewable energy.

AIM-listed firms such as Clipper Windpower, Ocean Power Technologies and Solar Integrated Technologies hope to harness the wind, waves and sun to generate cleaner energy.

Others such as Ceres Power hope to cut power usage through fuel cells, which make electricity by chemical reaction.

Its share price has more than doubled to 241 pence in less than a year on the back of technological advances and a deal with British Gas, owned by Centrica Plc.

British company Alkane Energy Plc captures gas trapped in mines and landfill sites or given off by animal dung for use as a renewable energy.

Its Chief Executive Cameron Davies said the risk of investing in growing companies in the green sector is outweighed the growth potential.

But he played down talk of the next Google emerging from the sector as "far-fetched".

"The thought of a company coming along with a magic idea is pretty unlikely," he said. "It's more evolution than revolution."

With green issues high on the political agenda, smaller firms could attract more investment.

Prime Minister Tony Blair warned this month it was an "absolute necessity" to confront climate change, while US President George W. Bush said in Febraury that "America is addicted to oil".

But the green sector may lose its admirers if politicians look to nuclear power or if oil prices fall.

"Long-term prospects are pretty compelling, but for mainstream investors it's too high risk," said Mark Hinton, research analyst at the financial advisers Bestinvest. Norwich Union, which has a "green" fund run by Morley Fund Management, said last month demand for cleaner fuels will rise, but investors should be cautious.