Licensees are still breaking the law by passing off cheap spirits as popular brands, despite a major crackdown by trading standards officers (TSOs).
The warning follows an investigation by TSOs in Birmingham, where officers were shocked that, despite their heavy presence in the city, a minority of publicans are still taking part in the illegal practice.
Trading standards spent two months in the city and out of the 251 outlets investigated four per cent were found to be selling at least one fake brand.
In one case as many as nine spirits had been substituted.
The main problem is from "tipping" where licensees pour cheaper spirits into branded bottles and sell them on at the premium price to customers.
Those who are caught doing so face criminal proceedings, heavy fines and could lose their licence.
"The majority of pubs and bars in Birmingham are completely straight," senior TSO Jack Yates said.
"However, there are a few rogue licensees out there who think they can get away with duping the public into paying for a brand of spirits that has been switched for a cheaper, lesser quality version. We aim to stop this happening."
Meanwhile, bootlegged alcohol is making the problem much worse.
John Treadwell, who was responsible for the survey, warned licensees against taking up offers of cheap booze sold from the backs of vans since this is almost certainly brought into the UK illegally by bootleggers who evade paying duty.
He said: "Many publicans reported being offered cheap spirits from passing white vans.
"This is most likely to be smuggled stock from Europe and it is just as well that these offers are rarely taken up by licensees as the resale of bootlegged alcohol is more likely to lead to imprisonment than a fine."
The International Federation of Spirit Producers is backing a widespread crackdown on "tipping" off after schemes in Manchester and other parts of the country reported that incidents of substitution had halved over the last two years.