An undercover investigation in Britain's pubs has discovered that as many as one in 12 bottles of branded spirits contains cut-price substitutes.
The investigation was possible using new technology that meant trading standards officers could test spirits using specially developed dip tests.
Up to eight per cent of licensees investigated were found to be cheating customers by filling top brand bottles with cheaper product in order to boost profits - a practice that is illegal but is thought by trading standards officers to be widespread among unscrupulous publicans.
Until recently there was no easy way to prove that pubs were passing off cheap spirits as well-known brands, but last year some of Britain's spirit manufacturers agreed to add small amounts of marker chemicals to their products.
Trading standards officers were given kits to test drinks in pubs nationwide and the results, which have just been published, suggest that thousands of pubs, bars and other licensed outlets have been involved in the illegal scam.
The trading standards investigation focused on the best selling white spirits in Britain - Gordon's gin, Bacardi rum and Smirnoff vodka.
The testing kits contain a variety of plastic sticks, each one for testing a different brand. The sticks are impregnated with a substance that changes colour when it comes into contact with the chemical that has been added to a particular brand.
So when a stick designed to test Gordon's gin is dipped in the real brand it changes from green to yellow, but if it is dipped in a cheap substitute it turns blue.
In one county alone, 1,000 pubs were investigated and more than 85 were found to be topping up spirits with cheaper alternatives.
The results of the chemical tests are not enough for trading standards officers to prosecute licensees, but they do mean they can take samples away with them for further testing.
So far hundreds of licensees have been warned or prosecuted by trading standards and, as word of the new test has spread, the numbers of publicans passing off cheap spirits as top brands is thought to have fallen.
Now the only problem left for trading standards officers is to find a test for Scotch Whisky. Because the manufacturing of the spirit is strictly controlled, adding marker chemicals to it is currently forbidden.