A fishmongers in Hertfordshire has found a loophole in the law which allows it to sell fish in pounds and ounces without facing prosecution.
Customers must pay a penny to join the private club
Gemini Fish Supplies in Codicote, near Hitchin, is officially a private club and can trade in imperial units.
Joint proprietor Irene O'Brien said
half their customers did not understand the metric system.
Under an EC rule used in Britain it is illegal to weigh or advertise a product using imperial measurements alone.
Ms O'Brien, who has run the shop with Peter Halstead for the past 27-years, said: "We just had a gut feeling that because we were a club the rules would be different and we've been told that they are.
"We're not planning to overthrow the government, we're just campaigning for customers' rights.
"We've had customers coming from as far as Leighton Buzzard because they feel so strongly about the issue."
Metric units are now generally required to be used for purposes of trade, but imperial units may continue to be used alongside their metric equivalents until the end of 2009.
In 2001 Steve Thoburn, a Sunderland grocer, was convicted of selling bananas only by the pound, under an amendment to the 1985 Weights and Measures Act which stipulated that customers must be told the metric weight of all loose food.