First the good news: today's cars are more efficient and cleaner than ever; there is more incentive to choose an energy-efficient vehicle; and there is a range of fuels that mean less damage to the environment.

Now the not so good news: the genuinely green, pollution-free car is still about 20 years away, and the number of cars choking our roads is still proliferating at a depressing rate. For the time being, at least, the automobile remains environmental public enemy number one.

It can all be very confusing if you need to drive a car, but want to do so with as clean a conscience as possible.

The search for a replacement to the petrol-guzzling, exhaust-belching internal combustion engine has become a sort of holy grail for car manufacturers in recent years. The hot money is now on hydrogen fuel cell technology.

The principle is simple: with the help of a catalyst, hydrogen is combined with oxygen in the air to produce electricity that drives the engine, with water vapour the only emission. What's more, it works, as a number of test and production vehicles have demonstrated.

The problem lies in the production and distribution of hydrogen. As Robin Oakley of Greenpeace says: 'There is a time gap, both for the fuel cells to be fully developed, and to have hydrogen available on forecourts. It will require a large amount of investment.'

More to the point, he says, hydrogen is only green if from renewable sources.

The most straightforward way to produce hydrogen is by using electricity to separate water; but that makes sense only if the electricity comes from wind farms or other renewable sources. The same issue arises with cars running on rechargeable electric batteries. Although they don't emit noxious fumes and CO2 in city centres, the pollution is simply being caused elsewhere in electricity production. Electric cars also have a limited range.

So what can the conscientious motorist do today? The first answer is that, before you even start to choose a vehicle, you should think about changing your lifestyle to use your car less.

Andrew Davis of the Environmental Transport Association, the body behind next month's Green Transport Week, says: 'Look at where you live and where you work, where you shop and where your children go to school.'

The savings to be made from living in the right place are much more significant than those from choosing the best model of car, he says. People can also drastically reduce the impact of their driving on the environment by planning journeys more carefully, driving less aggressively and servicing their vehicle properly.

When it comes to choosing a car with the environment in mind, there are two main considerations: how efficient is it; and how clean are its emissions?

The latter point is covered by European standards. 'The Euro IV standard comes into force in 2006, and imposes much stricter controls on the particulates and other pollutants that a car can emit,' says Nikki Rooke of the Society of Motor Manufacturers and Traders. 'But there are already many models available in the UK that are Euro IV compli ant and there will be many more in the next 18 months.' Efficiency matters because cars are reckoned to produce some 25 per cent of the UK's output of CO2, the main contributor towards global warming.

The most efficient car on the UK market is a hybrid petrol/electric model, the Honda Impact. This type of vehicle harnesses the frictional energy created when a car slows down to generate electricity, which is then used to top up the power when the car accelerates.

With a CO2 emission of just 80g per kilometre, the Honda Impact is twice as efficient as the average car, but at £17,000 it has sold in limited numbers and is available only by special order from Japan. Honda does have another vehicle using the same technology, the five-seater Civic IMA at £15,000, which produces roughly 25 per cent less CO2 than its peers at 116g per kilometre.

Toyota's Prius, at 104g per kilometre - 364 were sold in the UK last year - is also among the most efficient on the market. Both qualify for a grant of £700 towards the purchase cost from the Energy Saving Trust under its Powershift initiative.

You can also get Powershift grants to convert a conventional car to more efficient fuels, such as LPG or natural gas. Another incentive is that these fuels, as well as bio-diesel - a mix of 95 per cent diesel with vegetable oil - carry less duty, and so cost less to run.

However, rescue workers in the US are already voicing concerns that the hefty batteries and cabling may cause safety problems when extracting people from this type of car. And a number of low-emission vehicles qualify for a 100 per cent discount against the congestion charge, which could save more than £1,000 a year if you drive into central London every day.