Gang 'cheated casino out of £55,000 by using special contact lenses to see invisible ink marked on cards'
And even suspicious staff, who have seen all the tricks in the book, were unable to work out how the group of Italians had beaten the house again.
But eventually their luck ran out when police caught onto their sophisticated scam.
By wearing special contact lenses to see invisible ink on marked cards, a gang is said to have conned the cash out of casinos across Europe.
But the alleged ruse was rumbled this summer in the French city of Cannes when casino staff reported their antics, after becoming suspicious of their £38,000 and £17,500 wins on two separate days.
A police spokesman said: 'We have never seen anything like this in the region. At first we thought that they were using cameras.
'But we didn't find anything. Finally we realised that their strategy involved using contact lenses.'
Detectives now believe the gang may have used the technique in casinos across the Continent before their arrest in Cannes.
The alert was raised when three Italian players won almost £38,000 in an evening playing a form of stud poker against the croupier in the Princes Casino on the French Riviera.
Claims: The Princes Casino, in Cannes, was allegedly targeted by the contact lens gang
A week later they made a further £17,500 within a few hours. But staff recognised them and called police.
Further investigation revealed invisible marks on the back of the cards being used. A cross indicated a king, a line meant an ace - information that gave the three an unbeatable edge.
Grasse state prosecutor Jean-Michel Cailleau said the men were all wearing treated contact lenses so they could read the invisible ink.
The three Italians, and two alleged French accomplices, are being held on suspicion of 'committing fraud in an organised gang' and face a maximum sentence of ten years in prison if found guilty.
One of the Frenchmen arrested is believed to work for the casino and police believe he may have slipped the marked cards into the venue.
Police are now going to send mugshots of the suspects to casinos across the continent.
Maitre Jean-Louis Paganelli, a lawyer for one of the Italian players, who have not been named, said that his client denied any wrongdoing.
He told the Times: 'He doesn't understand why he has been arrested.
'He came to the Cote with a friend to have a good time and only brought a pair of sandals, a towel and a pack of cards, but that doesn't make him a cheat.'