The speed limit may be cut from 60mph to 50mph on thousands of rural roads without any change to signs, leaving unsuspecting drivers at risk of prosecution.

Ministers are looking at the change for “non-built-up single-carriageway roads”, which account for more than half the network. The national speed limit sign — a white circle with a black stripe - would mark the beginning of a 50mph limit for cars, rather than 60mph as at present.

The lower limit would apply automatically unless the local authority could prove that it was safe for the road to remain at 60mph.

The Department for Transport (DfT) said research had shown that the change would save up to 250 lives a year. It also favours a lower limit because it would reduce fuel consumption and carbon emissions.

Drivers are three times as likely to die per mile driven on rural roads as on urban roads. There were 1,603 deaths on rural roads in 2007 compared with 1,160 on urban roads.

However, road safety and motoring groups said that a lower limit would make little difference to safety unless it was accompanied by greater enforcement and changes to the layout of rural roads, such as improving visibility at junctions.

The change may have no impact on average speed, which DfT surveys show is only 48mph in free-flowing conditions on rural 60mph roads.

Robert Gifford, director of the Parliamentary Advisory Council for Transport Safety, said: “Drivers are still killing themselves despite driving at less than 50mph. Lowering the speed limit on its own is not going to save many lives.

“We need engineering measures to help drivers read the road better. There is also a risk that drivers will not realise that the limit has changed because there are no new signs.”

An AA survey has found that half of drivers do not understand the national speed limit sign.

Edmund King, president of the AA, said: “There are many single-carriageway roads where 60mph is appropriate but local authorities may not bother to make the case for keeping the existing limit. This will lead to frustration and could raise the risk of dangerous overtaking of law-abiding drivers doing 50mph.

“Many crashes on rural roads happen not because the speed limit is too high but because reckless young drivers lose control.”

The Institute of Advanced Motorists said: “A blanket 50mph speed limit will not command respect unless drivers can see a clear link to crash risk. A detailed review which assesses every mile of rural road is needed.”

A DfT spokesman said: “Any such proposal would have to be based on robust evidence of the impact on casualties, emissions and journey times. It would also need to consider issues of enforcement and public acceptability. If such proposals were ever taken forward, it would mean a redefinition of the national speed limit on the roads in question rather than requiring additional signage.”

The Conservatives rejected the idea of a blanket reduction in the speed limit. Theresa Villiers, the Shadow Transport Secretary, said: “We believe that a targeted approach is more effective, leaving local authorities to take the key decisions on whether reduced speed limits are the best option ... Rather than across-the-board reductions in the speed limit that hit everyone, including the safest and most responsible drivers, we believe that a successful strategy to make our roads safer needs to target problem drivers.”