Motorists could find that their car insurance policies are invalidated if they have poor eyesight, it has been claimed.

A survey from Specsavers shows that one in four drivers cannot correctly make out a vehicle's number plate from 20.5 metres - even if conditions are clear.

The research was carried out by qualified opticians on 160 motorists at this year's Royal Highland Show.

Specsavers representitive Shirley-Ann Kennedy remarked that motorists can be punished if they take to the road with less than adequate eyesight.

"Many drivers still don't realise that if they can't read a number plate from a distance of 20.5 metres or five car lengths in good light, they are committing an offence which may invalidate their car insurance," she said.

Many of the offending motorists are thought to be the elderly whose sight has deteriorated over time. The law states that drivers who need glasses or contact lenses must always wear them while driving and driving with uncorrected vision is an offence carrying a fine of 1000 and up to three penalty points.