The motorway speed is set in increase from 70 to 80 mph in 2013 under a new government plan to help boost the economy.

There are also plans for a big expansion in the number of 20 mph zones in cities and towns, Transport Secretary Philip Hammond has announced.

Hammond has argued that the faster speed limit will help to boost economy by shortening travel times.

The policy is part of Tory plans to win the "Top Gear vote" and Hammond said Thursday: "The speed limit is nearly 50 years old, and out of date thanks to advances in motoring technology. Increasing the motorway limit to 80 mph would generate economic benefits of hundreds of millions of pounds through shorter journey times."

But the move is likely to be criticised by environmentalists, who argue that cars use about 20 per cent more fuel at higher speeds and warn of an increase of deaths and serious injuries on the road.

"Drivers travelling that 10 mph quicker might reach their destination sooner, but will use about 20 per cent more fuel and emit 20 per cent more CO2," said the head of the RAC Foundation, Professor Stephen Glaister.

"There is also likely to be a slight increase in road casualties. And what about enforcement? If police follow existing guidelines, many people could do 90 mph before action is taken.

"Local authorities already have the power to set 20 mph limits, and that's how it should stay.

"Before you change a speed limit, you have to know whether you are doing it for safety, economic or environmental reasons. Unfortunately not all of these are compatible."

The existing 70 mph speed limit was set in 1965. The government has said that cars are much safer now than they were then, with a fall of 75 per cent in the number of people killed every year on British roads since then.