(CNN) — You've just spent a barrel of cash on it, but is that very expensive whisky actually good to drink?

With experts warning that a third of rare Scotch whiskies tested in a laboratory have been found to be fake, it could leave a bitter aftertaste.

Twenty-one of 55 whiskies analyzed -- estimated to have a combined value of £635,000 ($800,000) -- were discovered to be modern fakes, according to the group Rare Whisky 101 which sent the samples for testing.

The results suggest that around £41 million ($52 million) worth of rare whiskies currently circulating in the secondary market are fake, the group said.

The analysis, conducted at the Scottish Universities Environmental Research Centre, was arranged due to growing concerns in the industry about counterfeit whiskies in the secondary market.

Three rare bottles of single malt whisky which were proven to be fakes Iain McLean

"The exploding demand for rare whisky is inevitably attracting rogue elements to the sector," Andy Simpson, co-founder of Rare Whisky 101, said in a statement, urging buyers to ensure whisky they purchase is authentic.

Whiskies purporting to be more than a century old were especially likely to be created by scam artists, with every malt whisky sample supposedly from 1900 or earlier found to be fake.

"It is our genuine belief that every purported pre-1900 -- and in many cases much later -- bottle should be assumed fake until proven genuine, certainly if the bottle claims to be a single malt Scotch whisky," the group's co-founder David Robertson noted.

"This problem will only grow as prices for rare bottles continue to increase," he said.

The prices which the bottles could have fetched at auction -- had they been genuine -- range from £2,500 ($3,200) to £150,000 ($190,000).

The UK's secondary whisky market has grown in recent years, with its value currently standing at £36 million ($46 million), according to the organization.