LONDON - Britain should ban all drinking before driving to end uncertainty about what the law allows, 40 percent of motorists who drink alcohol said in a poll on Tuesday.

Forty-five percent admitted having driven after drinking alcohol and four percent had done so frequently.

The survey for Direct Line Car Insurance also showed that one third of motorists do not know how many units of alcohol are in an average strength pint of beer or how that may affect their blood alcohol level.

Almost half were ignorant of the potency and effects of a large glass of wine.

Direct Line said the confusion over how much alcohol is "safe" to drink before driving, arises from the fact that alcohol intake is measured in units, while the drink/drive limit is measured by alcohol content within the blood.

One pint of ordinary lager, bitter or cider, or a 175 ml glass of wine equal two units. The UK legal limit is 80 milligrammes of alcohol in 100 millilitres of blood.

The blood alcohol level can be affected by such factors as an individual's size, weight and metabolism, meaning there is no uniform measure.

"Findings from our study make worrying reading," said Tony Chilcott, head of car insurance for Direct Line.

"If an average-sized female motorist drinks two large glasses of wine during an evening out, then that is the equivalent of two-thirds of a bottle of wine.

"Whilst she may then feel 'fine' and wrongly assume that she has only had two units of alcohol, should she then drive home, she is extremely likely to be over the drink drive limit.

"It is this uncertainty that many motorists want to end by banning all drinking before driving."

The YouGov poll was carried out among 2,066 adults in November.