The British government is planning to cut the national speed limit from 60mph to 50mph on most of Britain’s roads, enforced by a new generation of average speed cameras.

The reduction, to be imposed as early as next year, will affect two thirds of the country’s road network. Drivers will still be able to reach 70mph on motorways and dual carriageways and 60mph on the safest A roads.

Jim Fitzpatrick, the roads minister, defended the plan, which will be the most dramatic cut since 1978, when the national speed limit was reduced from 70mph to 60mph.

“There will be some in the driving lobby who think this is a further attack and a restriction on people’s freedom,” he said. “But when you compare that to the fact we are killing 3,000 people a year on our roads, it would be irresponsible not to do something about it. I’m sure that the vast majority of motorists would support the proposals.”

New research by the Department for Transport has found that reducing the speed limit could save 200-250 lives a year and also reduce carbon emissions.

Britain’s roads were the safest in the world until 2001, relative to its population, but have since fallen into sixth place behind countries such as the Netherlands, Sweden and Norway. Some challenge that statistic because of the disparity of the countries’ sizes.

The new 50mph limit is intended to reduce the high death toll on rural roads, where, in 2007, 69% of car crash fatalities took place. It will apply to single carriage A, B and C roads. Local authorities will have the power to raise the limit to 60mph on the safest roads, but will have to justify it.

Ministers plan to use average speed cameras, which monitor speeds over distances of up to six miles, to help enforce the new limit. The cameras have already been installed at 43 locations. The Home Office is expected to approve their wider use later this year.

Speed Check Services, the company behind the cameras, claims the number of deaths or serious injuries at its sites has fallen on average by 60%.

Fitzpatrick said: “If you look at the figures on rural roads, there are disproportionately more people dying there than on any other roads. The nature of some rural roads, with dips and bends and difficult conditions, means that the 60mph limit is not enough.”

The 50mph proposal will be laid out in a consultation document to be published in the early summer.

Edmund King, president of the AA, warned that the move could alienate some motorists. Last year the AA asked 17,481 motorists if the limit on single carriageway roads should be cut to 50mph. Nearly half backed the move but 38% opposed it.

He said: “There are quitea few single carriageway rural roads that are straight and adequately wide, where 60mph – in the right conditions, driving sensibly — is not a problem.

“The danger of the blanket approach is: are you going to then reduce speed limits just for the sake of it where you don’t need to? That’s where you lose the respect or the support of the motorist.

“We all know some rural roads where the 60mph limit is ridiculous, although there are equally others where it suits. So it is a case of getting that balance.”