Almost a million patriotic drivers have attached flags bearing the St George cross to cars and vans. But motorists have now been warned that their show of support may breach road safety regulations.
Badly positioned large flags could obscure windows and lead to a potential road accident.
Under the Road Traffic Act of 1998, drivers who obscure the windows of their vehicles are liable for a fine or points on their licence.
More serious charges could be brought if pedestrians or cyclists are injured by flags sticking out the side of vehicles.
Sainsbury's an official sponsor of the England team in Euro 2004 has sold over 75,000 car flags in a pack which includes an England bumper sticker.
Asda has sold more than 30,000 and other supermarkets also share similar sales figures for flag and sticker packs.
Motorists are being urged to make sure that their flags are secured properly before going on a journey.