Cheating pub and club landlords and bar tenders have been warned to beware when watchdogs arm themselves with a new detection weapon.

Nicknamed Robo-nose, the hand-held device can tell instantly by smell whether drinkers are being ripped off by cheap substitutes to top up bottles of brand name spirits.

If summer trials in West Sussex are successful, it will eventually replace kits used across the country, meaning samples no longer have to be sent off to laboratories for analysis.

Latest figures show one in 20 licensed premises in the county are ripping off their customers to the tune of an estimated £20 million a year.

Trading standards officers in West Sussex were given the chance to pilot the new monitor after being awarded third prize in a nationwide scheme to cut that figure, run by the International Federation of Spirit Producers.

The county team tested more than 900 spirit samples last year on undercover visits.

Chris Tinley, the county's trading standards manager, said: "At the moment we're using a small sensor that you dip into the drink. If it changes a certain colour, we take a sample of the spirit and send it off to the public analyst.

"It's a very costly exercise but two years ago, one in 12 establishments were substituting their drinks.

"We've managed to bring that down to one in 20."

He said: "Sometimes it's the landlord, sometimes its a barman who likes a drink and then tops up the bottle with something cheaper from a cash and carry.

"They used to just add water but customers can tell so now we're able to check very quickly."

He said the new computerised device will revolutionise spirit screening when it is brought in later this year, sponsored by the producers of big-name spirits like Smirnoff, Gordons, Teachers and Bells.

He said: "We're one of only three authorities in the country to be offered the use of this equipment which should enable us to crackdown even harder on this practice."

Patrick Bligh-Cheesman, head of trading standards, said: "Formal sampling is expensive and being able to screen test in this way enables our officers to make far more checks for a fraction of the cost."

Michael Dennis, Cabinet member for public safety, with responsibility for West Sussex trading standards service, said: "Members of the public deserve the highest degree of protection against fraud we can provide."