A DDos attack, of the kind that brought down the WorldPay system earlier this month, floods a website with computer-generated requests.

Now, some organised criminals are using the threat of inflicting such an attack, which can cost vast amounts of money to the company, as a means of extortion.

"Criminal syndicates operating from Russia have targeted large online payment systems that belong to gambling sites," DK Matai of MI2G, which monitors unauthorised computer hacking, told BBC World Service's Analysis programme.

"In some cases the criminal syndicates have made subsequent phone calls and said, 'look, you have to pay us $40,000 or $50,000 before we will stop mounting these DDos attacks. If you don't pay us, then be ready for another day of disruption for your customers'."

"Some companies, because they are making more than $50,000 per week, have agreed to pay that money."

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