The country's only playing cards manufacturer had been subtly marking its cards to help gamblers in the know cheat their way to a fortune.

Officials at Caesar's Casino in Johannesburg first suspected something when over a three-week period blackjack takings dropped by 11 per cent, with total takings plummeting by $333,000 in 19 days.

A winning hand at blackjack has a value close to but below 21

Security men began watching players closely and examining surveillance tapes.

At least five gamblers behaved strangely, either darting their eyes quickly over the deck or making bets of widely different amounts.

Investigators found that the patterns on the back of the ten, jack, queen, king and ace were almost imperceptibly different.

Each were found to have a tiny blank space inside a repeated floral pattern on the horizontal edge on the back of the card, visible to players before the croupier deals.

Over a year the equivalent losses across the whole industry would have reached some $10m, said Ernie Joubert, chief executive of Global Resorts which part-owns the casino near Johannesburg.

The company said it had received 4,000 marked packs.

Other casinos alerted

Executives believe information about the marked cards was sold to gamblers either for a flat fee or commissions on their winnings.

It was not clear exactly who was behind the operation and no arrests have been announced.

Protea Playing Cards has been the sole card supplier to all 22 of South Africa's casinos and others in southern Africa for more than a decade.

The company has not yet commented.

Investigations are now under way to find out whether other casinos are affected.

"People always thought ... that South African blackjack players were the best in the world," Mr Joubert said. "Now there may be another reason."

Casinos in South Africa generate revenues of more than $333m a year.