The government is about to spend thousands of pounds of taxpayers’ money on a string of unremarkable looking mid-sized hatchbacks it hopes will boost its green credentials.

Ageing Rovers and Vauxhalls are to be retired from the car fleet used to take cabinet ministers to meetings. Some will be replaced by the Toyota Prius, a petrol-electric hybrid that ministers believe will help the problems of global warming and dwindling oil reserves.

Sales of the Prius (meaning “before” or “formerly” in Latin) more than doubled between 2004 and 2005 in Britain and last year stood at more than 160,000 worldwide. It was voted European car of the year in 2005 because of its ingenious use of battery power to save energy.

Cameron Diaz, Scarlett Johansson and Leonardo DiCaprio are among a list of stars who arrived at the Oscar ceremony in a fleet of Priuses and the car’s bigger sister, the Lexus RX 400h, the first hybrid SUV.

Chris Huhne, the Liberal Democrat leadership challenger, has a Prius, as does the former Tory leader Michael Howard, and his successor David Cameron wants one. Spurred on by the success of the hybrid hatch, Toyota has announced plans to sell 1m hybrid vehicles annually worldwide by 2010.

But doubts are emerging about how effective hybrid technology is in reducing emissions and fuel consumption. Critics say many customers are being duped by environmental propaganda into buying cars that have worse fuel economy than rivals.

In the United States, where 35 times as many Priuses have been sold as in the UK (240,000 compared with 6,800) there is growing dissatisfaction. A study by Consumer Reports, a US consumer body similar to Which?, concluded that the Toyota’s claimed fuel consumption figure of 65.7mpg (combined) was unrealistic. In real-world driving conditions fuel consumption was closer to 40mpg.

More than half of American hybrid drivers say they are unhappy with their real-life fuel economy, nearly double the proportion of drivers of conventional vehicles (27%).

Online chat rooms carry many discussions about fuel economy that falls disappointingly below advertised levels. “None of you have ever achieved the claimed mpg figures Toyota publishes,” writes a disillusioned Prius owner having consulted other contributors to the chat room on the Toyota Owners Club website, while Sandra C of Bloomington, Indiana, who bought her Prius in spring 2005, told Consumeraffairs.com she had achieved “consistently around 34 miles per gallon”.

If Sandra is correct, this would make the Prius no more efficient than a string of conventional petrol cars such as the Renault Clio, Peugeot 206 or Ford Fiesta, and some diesel-engined 4x4s or MPVs, such as the Honda CR-V and Citroën’s Xsara Picasso 1.6 HDi, which has an official figure of 55.4mpg.

Even if the official 65.7mpg is correct, this is still no more than cars such as the diesel-engined Citroën C1 and C2.

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In Britain the first mutterings of discontent are beginning. Jack Swales, a fish and chip shop owner from Yorkshire, claims the best fuel consumption he gets from his Toyota Prius is in the mid to low fifties, the same as a small conventional-engined hatchback. “I really love the car but I’m not impressed by the fuel economy,” he said.

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