The Toyota Prius, the car favoured by celebrities wanting to flaunt a green conscience, is to become bigger and more sporty, with a larger engine, quicker acceleration and a top speed of well above 100mph. The changes – coming at the expense of fuel economy and carbon emissions – will be incorporated in a new version to be introduced at the Detroit motor show in January.

The decision risks denting the image of the Prius as one of the greenest cars on the road and will be seized upon by critics, who accuse the car’s owners of being smug and joke that the “R” is silent in Prius.

More than a million of the hybrid cars, which have a petrol engine and an electric motor, have been sold since the first version became available in Japan in 1997.

By 2001 the Prius had started to become fashionable among Hollywood stars such as Cameron Diaz and Leonardo DiCaprio. The latter described it as a “great solution for the planet” and once owned four.

More than 28,000 have been sold in Britain, including several in the Government’s fleet of ministerial cars. In London, exemption from the congestion charge is as much of an attraction as the car’s green credentials.

Speaking at the Paris motor show last week, Miguel Fonseca, managing director of Toyota GB, said that the carbon dioxide emissions of the new Prius would be lower than those of the Mark 2, launched in 2003. But the reduction would not be as great as it could have been if Toyota had made environmental performance a priority.

“It will be cleaner, with CO2 emissions below 100gm per kilometre. We could have gone lower but, instead, we have chosen to give the Prius better performance.”

A Toyota source said the engine size would be increased to 1.8 litres, up from 1.5 litres on the existing model. The top speed is likely to grow from 106mph to about 120mph and the car will also be a few inches longer and about an inch wider. There will also be an improvement in the acceleration, which is currently 0-62mph in 10.9 seconds.

Toyota appears to be trying to broaden the appeal of the car, particularly in America, where drivers are used to having much larger, more powerful engines.

A Toyota spokesman said: “The customer base is big enough now that the Prius is not just appealing to the green niche.

“In the design phase, a decision is made about what we think the market is looking for in terms of fuel economy and in terms of performance, and how you make a compromise between these conflicting aims. We can tune the engine either way.”

Jay Nagley, a motor industry analyst, said: “It is ironic that, with the public now clamouring for the most fuel-efficient cars, the vehicle most associated with environmental awareness and fuel economy is getting higher performance.

“Toyota is displaying an old-fashioned mindset by thinking it has to make the new Prius bigger and faster to attract more buyers.”

Mr Nagley said that many Prius customers had found that the existing model performed far less fuel efficiently than the official consumption figure of 65.7mpg. In a test by cleangreencars.co.uk, a Jeep Patriot 4x4 and a Prius were driven between London and Brighton. The website found that they achieved 38.9mpg and 39.9mpg respectively.

“The gap between official mpg figures and actual performance is much worse for the Prius than for other cars,” he said.

Stephen Glaister, director of the RAC Foundation and a Prius owner, said that the existing model had plenty of power.

“The acceleration is very respectable and it does 80mph on motorways quite adequately. The engine does get a bit noisy above 70mph, but I would have preferred the new version to focus on improving fuel economy rather than raising performance.”

Toyota is working on a plug-in version that can be charged up from the mains so that owners would not need to use any petrol for short journeys.