By Alex Kirby BBC News Online environment correspondent

The best way to make the UK's road transport green could be a massive tree-growing programme, researchers say.

They say there is considerable potential for producing hydrogen and alcohol fuels from fast-growing trees like willows.

A quarter of all the UK's agricultural land would be enough to fuel the country's entire road transport sector, they believe.

You'll get a bigger bang for your carbon buck by taking out old, inefficient power stations Tim Brown, National Society for Clean Air

But they say it will be several decades before hydrogen is a sensible choice as a transport fuel.

The researchers are from three think-tanks: the Energy Saving Trust, the Institute for European Environmental Policy, and the National Society for Clean Air (NSCA).

They have produced a report, Fuelling Road Transport - Implications for Energy Policy.

Tackling the problem

In it they argue that the rapid expansion of hydrogen as a fuel for transport could in fact damage the environment rather than help it.

This is partly because electricity is needed to produce hydrogen from a source containing carbon, and there is a net loss of energy in converting it for use in motor vehicles.

But another reason, the report says, is because there are greater savings in emissions to be made by using electricity from renewable sources to replace old power stations.

Biomass means breathing easier

One of the authors, Richard Mills, said: "There is no doubt that, long-term, the transport sector could use substantial amounts of hydrogen from renewables.

"But in the medium term hydrogen will come from natural gas. It would make more sense to burn that gas directly in vehicles.

"A premature 'dash for hydrogen' could have an environmental downside, which can be avoided by encouraging the more efficient use of petrol and diesel hybrid technologies, and developing transport fuels from biomass."

Quicker returns

Tim Brown, of the NSCA, told BBC News Online: "You'll get a bigger bang for your carbon buck by taking out old, inefficient power stations."

The report says there is enormous potential for using biomass - vegetable matter - to make fuel for road vehicles.

It says: "Biomass offers a cheaper and earlier route than renewable electricity to reducing carbon emissions via a hydrogen-fuelled transport system.

"As an indication of the potential contribution, 25% of UK agricultural land planted with indigenous wood crops converted to methanol, ethanol or hydrogen could in the long term satisfy most or even all UK road transport fuel demand.

"This outcome would, however, be dependent on relative costs and a large number of technical factors."

The authors stress they are not suggesting turning over a quarter of Britain's farmland to providing fuel, but simply pointing out how easy it would be to produce enough.

The report says there are "compelling environmental and strategic arguments for reducing the carbon intensity of the UK energy mix, and in particular the road transport sector, which is uniquely oil-dependent."