Up to a quarter of licensed premises in some parts of the UK have been found to have counterfeit alcohol for sale.
And Trading Standards officials, who carry out spot checks, say it is a growing problem.
Trade sources say fake alcohol is being made by organised gangs on an industrial scale.
Alcohol fraud costs the UK around £1bn a year in lost revenue, according to government estimates.
In a series of raids of licensed premises at the end of last year, 26% of outlets were found to be selling counterfeit alcohol in south west England, 17% were doing so in Manchester and around 10% in West Yorkshire. Information was unavailable for the rest of the country.
Over the past year there have also been several discoveries of illegal alcohol production and bottling factories across the country.
In October, Her Majesty's Revenue and Customs (HMRC) officers seized 25,000 litres of counterfeit vodka, along with bottling and labelling equipment in Cheetham Hill, Manchester.
"Criminal gangs deliberately spread out their distribution and sales networks to disguise their activities by duping the public into believing it is a 'small, local fiddle' and not the bigger international fraud across Europe and beyond that it really is," said an HMRC spokesperson.
The counterfeit alcohol that is sold in off-licences enters the retail market from the backs of vans across the country. Counterfeit alcohol tends to be rebottled wine, where cheaper wine is poured into a more expensive bottle, or fake alcohol made in illegal factories in the UK.
"It's a growing problem, which we are getting more and more intelligence about," says Trading Standards officer Linda Plested, "but the real problem is you don't know what's in it."
Some of the illegally made vodka has been found to contain high levels of methanol, which is used to make anti-freeze and some fuels. Drinking high doses of it can cause dizziness, breathing difficulties and even blindness.
"There is evidence that the counterfeiting of alcohol in the UK is being taken over by organised gangs, setting up factories and making alcohol on an industrial scale, which then gets shipped out to off-licences, pubs and clubs," says alcohol industry spokesman David Bolt. His trade group, the International Federation of Spirits Producers, works with alcohol brands to protect them from counterfeiting.
It is not always possible to know when alcohol is counterfeit, but there can be risks to health, according to Stuart Crookshank from HMRC Inland Detection, which oversees the detection and raiding of illegal alcohol operations.
"If you're playing with alcohol, it's a pretty toxic product anyway even under health and safety conditions and properly manufactured," he said.
"You do it in some backroom distillery where the conditions are absolutely dreadful and the product, you don't know where it's come from. It's positively dangerous."
Anyone with concerns about fake alcohol is advised to take the product back or to call Trading Standards to get it checked out.
BBC Radio 5 live joined Trading Standards during a series of raids, including a small off-licence in Staines, west London, in early February. Nine bottles of fake wine and a bottle of counterfeit vodka were found in the first few minutes.
Trading Standards says it often plans raids based on information from the Food Standards Agency and customer complaints about which brands of wine or spirits are being counterfeited.
Trading Standards says there is a database of small mistakes and discrepancies that is frequently updated. This enables them to distinguish between real brands of alcohol and fakes.
It warns customers to look out for spelling mistakes on the label, to check whether all bottles are filled to the same level and whether the label is straight. It also recommends looking at whether all the bottles of the same product look the same.
Even smelling the alcohol can help: counterfeit vodka often has a strong acetone smell, similar to nail varnish.