By Michael Gregson BBC World Service business reporter

Will our motorways soon be filled with the aroma of chip oil?

Many hundreds of vehicles are now using recycled vegetable oil in their tanks rather than the usual diesel or petrol.

Supporters say it is a cheaper and more environmentally friendly way of motoring.

But users of the fuel must still pay tax on it, and UK customs officials warn there are tough penalties for people are caught evading the duty.

The rising popularity of using vegetable oil as a fuel came to light in Wales some months ago.

Staff at the Wal-Mart subsidiary, Asda, knew something was up when the shelves of their Swansea store were regularly being stripped of cooking oil.

Sales had jumped more than 250% and store supervisor Nigel Clark struggled to keep up with demand.

"We noticed people were selecting trolleys full of oil," he said.

"It wasn't one or two bottles, everybody was coming in for a certain line, but it was mass bulk buying. Full trolleys straight out to the car and off."

Eventually curiosity got the better of the supermarket staff.

"It was actually just a colleague who went up to speak to someone to ask them about it; why are you taking all the oil? And they came back, they're taking the oil to put in their car," said Mr Clark.

"We couldn't believe it first. We thought it was a practical joke."

Grease lightning

The canny Welsh shoppers had found out that they could run their cars on the same stuff they use to cook fish and chips.

If we find you (evading tax), you will face stiff penalties Paul Gerrard, Customs enforcement officer

The scheme quickly became popular as cooking oil costs a fraction of diesel, although it is illegal to use it as car fuel if you do not pay tax on it.

Similar discoveries have been made by many across the world.

In Brazil alcohol from sugarcane does the job.

In South Africa, coal has been used.

And there is also an emerging market for legitimate bio diesel, with major companies going green - and cutting costs.

Expanding industry

Ebony Solutions, just south of Manchester, is the UK's first large scale bio diesel plant.

It has a capacity of 250,000 litres a week and household names among its clients for top quality e-diesel made from recycled cooking oil.

"The bio diesel that we produce is a sustainable fuel," said Stephen Whittaker, the founder of Ebony Solutions.

"It is not using any of the carbon resources to power our vehicles. It is totally recycled and it is biodegradable.

"In fact, if we spilled a tanker load of our fuel on a field it would just disappear in 6 months time - which wouldn't happen with petroleum products."

Diverse customers

Centura Foods, which makes Bisto gravy and Sharwoods ethnic foods, runs its lorry fleet on bio diesel.

The company is very pleased with the results.

"We think it is cleaner," said Transport Manager Brian Tong.

"Some would say there is a slight improvement to engine performance. Through our monitoring systems we can accurately measure what we use and as I say there has been no difference what so ever."

You do not have to run a fleet of lorries to benefit from bio diesel.

Across the UK, a network has emerged of enthusiastic bio fuel crusaders who run their cars on anything from duck fat to vegetable oil.

"The important part about the way we work is that it is the opposite in its commercial gesture to the way multinationals work," said enthusiast John Nicholson.

"With multinationals, you end up with a small number of very powerful, controlling and rather wasteful companies.

"We are a network of small. responsible, careful companies that meet local needs from local resources. It is totally the different way of working."

Tax demand

But there can be drawbacks to this do-it-yourself approach to filling your car.

All cars on public roads in Britain must pay tax on the fuel they use - even if exactly the same fuel is tax free when used for cooking.

Fuel tax evasion costs the UK government around 1bn euros (�700m) a year - and is taken very seriously by the authorities.

"If we find you, you will face stiff penalties," warned customs enforcement officer Paul Gerrard.

"You could lose your vehicles, you will certainly end up paying back all the revenue that you have evaded and you could go to prison."