Drivers who stick to shandies over the festive season could find themselves over the limit and banned from driving.

That's the verdict of trading standards officers after a survey across Yorkshire into the alcoholic strengths of the drink in pubs and clubs.

The findings show that the alcohol by volume (abv) strength of shandies varies widely and could lull drivers into inadvertently going over the drink-drive limit.

The results showed that two pints of shandy could be equivalent to one and a half pints of beer, putting the drinker in danger of being over the limit.

Martin Wood, chief officer at West Yorkshire Trading Standards Service, said: "There is no easy way to tell how much you can drink and stay under the legal blood alcohol limit. Blood alcohol levels are affected by many things such as gender, weight and how much you've eaten.

"Just because you are under the limit doesn't mean that your reactions and judgement are not impaired.

"This survey shows that even drinking shandy can pose a risk. The only sure way is to stick to soft drinks."

The majority of beers and lagers sold in pubs and clubs have an alcoholic strength of between 3.5 per cent abv and five per cent abv, which means a pint of beer or lager contains between two and 2.8 units of alcohol.

By drinking shandy, you would expect to halve your consumption of alcohol, so a pint of shandy should contain between one and 1.4 units of alcohol. But the Trading Standards survey found the proportion of beer and lager in a shandy reached as high as 93 per cent- and quite a few contained around 70 per cent of beer or lager in the mix. That means the units of alcohol in a shandy could vary from 0.8 units to two units.

Mr Wood said anyone who thought they could drink one pint of lager or beer and still drive safely might assume they could drink two pints of shandy and have drunk the same amount of alcohol. But two pints of shandy could be the equivalent of one and a half pints of beer.

Bradford Inner City Licensees Association member David Haigh said: "Shandies can vary, which is why we serve tea and coffee. That allows drivers to steer clear of alcohol altogether and to avoid paying extortionate amounts for soft drinks.

"It isn't just a problem at Christmas. It's an issue all year round and we would support this research."