There is no proof 20mph limits make roads safer, according to an official report which found half of all motorists ignore the restriction because they do not believe they will be caught.

A study commissioned by the Department for Transport found there was “insufficient evidence” to conclude there had been a “significant change” in the number of crashes and casualties following the introduction of go-slow zones in residential areas.

Just 51 per cent of motorists were found to have complied with new 20mph limits - a statistic likely to raise major concerns among road safety campaigners.

Drivers said this was largely because of the lack of enforcement of the speed restriction, with the report blaming a “widespread view” that the chances of being caught were “very small”.

The report said the Department for Transport should update guidance to local authorities to encourage them to make 20mph limits more effective and to address the view the limits were not enforced.

It said where proactive enforcement by the police was not practical, councils should consider other methods to make motorists slow down - like the use of vehicle activated signs.

The law was changed in 1999 to allow local authorities to designate 20mph speed limits without needing the prior approval of ministers.