GORDON Brown's desire to have all UK motorists driving electric or hybrid cars by 2020 could be undermined by a shortage of the most popular hybrid models on the market.

The increasing cost of fuel alongside the heavily discounted level of road tax for vehicles such as the Honda Civic Hybrid or Toyota Prius has led to an unprecedented demand that has caught car manufacturers by surprise.

Car dealerships in Scotland are reporting at least one inquiry a day about buying a hybrid car - a type of vehicle that uses electrical power from a battery and a more traditional petrol or diesel engine to lower CO2 emissions and boost fuel efficiency.

However, waiting times for delivery of the new vehicles vary from three weeks to up to 20 weeks.

Campbell Chatham, who owns car dealerships in Edinburgh and Dunfermline, said that until recently, there was a six-month waiting list to buy a Honda Civic Hybrid. That waiting time was recently cut to 20 weeks, he said.

Chatham said: "Worldwide, there is a huge demand for these types of vehicles and we are seeing more and more people come into the dealership asking about them."

He said hybrid customers varied from individuals who "didn't want to have to explain to their neighbours why they just bought a new gas-guzzler" to businesses that want to promote an ethical image.

He added: "We have just had interest from the Scottish government, where one department is very close to ordering a whole fleet of hybrid cars."

Many Westminster departments now use Toyota Prius cars in their official fleets. Last week Gordon Brown, speaking at the G8 summit, championed hybrid technology.

He said: "I genuinely think this new technology - hybrids and so on - can be adopted widely to the benefit of the ordinary family, to change the family car and make it far more energy efficient."

But Paul Newton, from business analysis firm Global Insight, said Toyota was struggling to keep up because of a worldwide battery shortage.

He said: "This has been a big issue and Toyota is looking at opening a new battery factory in Japan to help boost supply."

Both the Toyota Prius and Honda Civic - the two most popular hybrid cars available - retail for around £16,000. Last year, Toyota sold 281,000 of the cars worldwide - a figure expected to grow to 320,000 this year. In the same period, 3000 Civic Hybrids were sold - a number Honda expects to increase significantly when it launches a cheaper version of the vehicle next year.

Depending on the type of vehicle, owners of hybrid cars receive a huge reduction in road tax. Payments can drop from £400 a year to as little as £35.

A Honda spokesman said: "The Civic hybrid is a global car and there is huge demand for it around the world. Right now we are just not able to meet the demand."

Michael Velvo, a Toyota spokesman, also admitted there was a shortage. He said: "The demand for these cars is just sky rocketing and we are being stretched beyond full capacity. It's not that bad a position to be in really, but we are doing all we can to increase supply, including opening a new plant in Japan to help increase battery supply."

Not all dealers, however, are convinced of the hybrid car boom. Brian Gilda, chairman of car dealership chain Peoples, said he did not agree with the idea that demand was rocketing.

He said: "We are not finding that people are coming in and asking for these cars. There is not a clamour for it, but I expect that to really start happening at some point in the next three to five years. What we are under pressure to deliver now is a more economical vehicle and we are finding a lot of people are downsizing to a smaller car than they had previously."