Bar managers have obtained bulk supplies of cheap cola and sold it from draught taps marked with the big-brand names such as Coca-Cola and Pepsi-Cola.
The colas being passed off as the real thing have cost less than one-third of the wholesale price of leading brands - but customers have paid a full mark-up price.
The ploy was uncovered in West Glamorgan, where trading standards officers found problems with the sale of soft drinks in six of the eight clubs and public houses that they checked.
The Coca-Cola company said that the problem could be more widespread. 'I suspect this sort of thing goes on from time to time,' Ian Muir, a company spokesman, said.
He added that the company had taken action against some cola suppliers in the North-west to prevent a similar problem.
The British Soft Drinks Association said: 'Soft drinks piracy is a serious problem. It damages the consumers' perception of the product. They will not be buying products to the recipe and quality established by the company and the brand's reputation will be affected.'
John Richmond, West Glamorgan's assistant county trading standards officer, said that three of the region's bars where customers had been sold fake colas had already pleaded guilty to the offence in court.
Another case was due to be heard next month and investigations in two more bars were continuing. 'There is nothing illegal in buying a cheap substitute and labelling it as such,' Mr Richmond said.
'But it is an offence to represent it as something else. A lot of customers did not know the difference.'
The Pepsi-Cola company said that it was aware of the action by trading standards officers in West Glamorgan, but the company did not know of similar problems anywhere else.