Spirits manufacturers are adding a secret ingredient to leading brands to help trap pubs, clubs and restaurants using cheap substitute spirits to rip off drinkers.

Spirits manufacturers are adding a secret ingredient to leading brands to help trap pubs, clubs and restaurants using cheap substitute spirits to rip off drinkers.

Almost one in 10 licensed premises are cheating customers by serving cheaper brands than those on show behind bars or advertised on menus, say trading standards officers and spirit producers. Landlords and restaurant owners are refilling empty bottles of well-known brand-name spirits with lower quality substitutes from supermarkets and pocketing the difference in cost.

There is also concern that motorists are being unwittingly pushed over the drink-drive limit by being served substitute spirits smuggled in from the Continent. These have a higher alcohol content than those usually on sale here, sometimes by up to 20 per cent.

In an effort to beat the spirit swindlers, some makers of famous names are including an additive in their drink that can be detected by a simple test. Smirnoff vodka and Gordon's gin are already doing so, and Bacardi rum and a number of other major brands are considering following suit. The sugar-based additive has no taste or smell but its presence can be identified within 30 seconds by dipstick testing strips which are being issued to every trading standards service.

The industry estimates that at least 10,000 of Britain's 130,000 licensed hotels, pubs, clubs and restaurants are selling low-quality spirits under famous names, and that owners will be looking to cash in on the busy festive period. With supermarket own-label spirits costing about half that of the major brands, unscrupulous landlords can make thousands of pounds each year by refilling bottles with cheaper alternatives.

White spirits are substituted most often because they are usually mixed with orange juice and cola which the licensee hopes will disguise the taste. Whisky is less likely to be substituted as it is frequently drunk neat and there is a greater likelihood of customers tasting the difference.

"The landlords probably think that the customers won't mind because it is the same brand," said Terry Carter, principal officer with Buckinghamshire Trading Standards. "But the imported spirits can be quite a bit stronger and this can be significant enough to push drivers over the limit. We wonder when we will have the first death because a driver has been put over the limit by a landlord serving bootleg spirits."

The International Federation of Spirit Producers has launched a Christmas and New Year campaign to tackle the problem, which it believes is costing drinkers £10m a year. Philip Scatchard of the IFSP said: "There are a growing number of rogue operators who think that they can con consumers by pouring cheaper spirits back into branded bottles and passing them off as the real thing. A significant percentage of outlets are determined to turn every trick in the trade to make extra, illegal profits and defraud their customers."

Customs and Excise officers also fear that drinkers are also at risk of being served dangerous illegally distilled spirits. Officers uncovered an illegal still in a raid on a West Midlands industrial estate which had 10,000 bottles ready to be filled, and estimated that almost 10,000 litres had already been produced and sold.

"Any 'home-distilled' spirit is not going to be pure. It will contain impurities that are potentially dangerous as they could be poisonous," warned a Customs and Excise spokesman.

Even more worrying was the discovery of a large consignments of methylated spirits labelled as vodka and whisky seized by customs officers at Dover and in the North-east last year. It is believed that the methylated spirits had been produced and bottled on the Continent and then imported into Britain.