Coffee drinkers are being conned by suppliers fraudulently mixing inferior beans into products labelled 100% Arabica, scientists say.

A study by British researchers testing a new and more accurate method of testing coffee quality looked at coffee on sale at shops and supermarkets.

They found that a tenth of high quality products labelled "100% Arabica" contained significant levels of inferior and cheaper "Robusta" beans. Arabica coffee trades at twice the price of Robusta because of its superior taste.

Finding rogue Robusta in a sample labelled Arabica is not easy, especially after grinding and roasting.

The standard technique detects the fingerprint chemical 16-OMC, which is only found in Robusta coffee, but is costly and takes three days, making large scale surveillance impractical.

The new method takes only 30 minutes and is sensitive enough to detect just 1% Robusta in a blended coffee.

Lead scientist Dr Kate Kemsley, from the Quadram Institute, formerly known as the Institute of Food Research, said: "This is an important milestone for detecting fraud in coffee, as 1% is the generally accepted cut-off between trace contamination, which might be accidental, and more deliberate adulteration for economic gain."

For the study a total of 60 different coffee samples were purchased in consumer countries around the world, including 22 from the UK.