Hundreds of thousands of football fans are flocking to South Africa for the World Cup, which begins 11 June. But for banks, the world's biggest sporting event won't just offer thrilling goals and hard-fought games. It also brings major worries about credit card fraud.

Credit card experts warn that banks face a huge double challenge ( Criminals are expected to target football fans during the month-long event, stealing their credit card numbers and racking up big purchases. So banks must be quick to crack down on suspicious card use. Yet at the same time, banks have to be careful not to cut off the cards of tourists and fans whose legitimate purchases will mistakenly trigger fraud alerts. That would lead to angry fans relations nightmares.

Here's why. Current fraud detection technology works by comparing any new card use with a cardholder's historical patterns of spending. If a card that's been used for years only in London is suddenly buying drinks in a bar in Cape Town, South Africa, well, that's suspicious.

"But this approach has two big limitations. It may take several of these doubtful purchases to trigger the fraud alert. By then, criminals may have gotten away with thousands of dollars of goods." says Pat Carroll.

Worse, 9 out of every 10 cross-border transactions flagged as fraudulent -- and thus rejected by the bank -- are actually legitimate. Many of the football fans at the World Cup will never have been in South Africa before. So their legitimate purchases in Cape Town or Johannesburg or any of the other seven cities where games will be played may be mistakenly tagged as suspicious. These so-called false positives cost banks up to $10 per case, since the bank's fraud unit must process each transaction and try to call the cardholder in an effort to verify its validity. That's a big cost. And fans trying to have a good time will be furious if their cards get cut off when banks can't reach them to verify transactions.

That's why fraud experts are telling banks to encourage their customers to inform banks about their travels to the World Cup, or to allow banks to call them on their cell phones to check on transactions.

The secret is using cell phone data to verify the legitimacy of a card transaction at the time of each purchase, but in an anonymous manner.

Each time a card is used, the software determines in real-time, that a physical transaction involving a payment card is in proximity to a mobile phone linked to the actual cardholder. If the cell phone is in the same place as the credit card purchase, then the transaction is probably legitimate -- even if the location and type of purchase represent a sharp break from the customer's historic spending patterns. That obviously will be true for thousands of World Cup fans. The whole process takes less than half a second.

Instead of declining legitimate transactions 9 times out of 10, the ValidSoft technology spots the real transactions correctly 95+% of the time, which in turn increases the fraud detection rate. "It turns the 90% failure rate on its head," says Carroll. Banks would save big bucks not having to verify legitimate transactions. Customers are happier too.