A gang has been caught red-handed in France using invisible ink and special contact lenses to cheat casinos out of vast sums of money.

In a turn of events that might have come straight from a Hollywood blockbuster, three Italians and two Frenchmen have been caught using special contact lenses that can detect invisible ink to cheat casinos out of huge sums of money.

The gang is believed to have used the method in casinos across Europe before attracting the attention of authorities in the French resort town of Cannes.

"We have never seen anything like this," said a police spokesman. "At first we thought they were using cameras, but we didn't find anything. Finally we realised that their strategy involved using contact lenses."

Detectives investigating found the gang used invisible marks on the backs of cards that could be detected using specially treated contact lenses. A cross indicated a king, while a line meant the card was an ace - giving the men a major advantage over other players at the table. It is believed that one of the Frenchmen, who worked at the casino, slipped the doctored cards into the deck.

The alert was raised when the players won €20,000 (£17,500) in a game of stud poker in the Princes Casino on the French Riviera this week. The gang was recognised by casino staff as the same men who had previously won €44,000 (£38,000) at the same table only days before.

Photographs of the suspects are to be sent to the Continent's leading casinos, say police, who believe they had probably been operating for some time.

The men face a maximum sentence of 10 years in prison if found guilty of fraud.

In 2004, three men scammed €100,000 from 32 casinos in the area by using a miniature camera hidden in the sleeve of one of the players to film the croupier. In the same year, a Hungarian woman and two accomplices got their hands on €1.8 million (£1.5 million) thanks to a mobile phone equipped with a device to measure the speed of the ball at the roulette table. The information was then transmitted to a microcomputer that calculated six possible numbers on which the ball would be most likely to land.