The £3.5 million figure was quoted by prosecutors during the first day of the trial of a 22-year old Northhampton University student at Southwark Crown Court.

Christopher Weatherhead is alleged to have been one of the ring-leaders in a string of DDoS attacks on financial services firms last year that were initiated in retaliation to the blockade of payments to the whistleblowing Web site WikiLeaks.

The operation Payback campaign targetted the Web sites of PayPal, Visa and MasterCard,among others, during a packet-flooding blitz in December last year.

The prosecution claims that Weatherhead was a key figure in the attacks, encouraging other hacktivists to join the campaign and boasting of its successes on an Internet Relay Chat (IRC) channel.

In calculating the £3.5 million cost, the prosecution says that Paypal suffered "considerable damage to its reputation and loss of trade", as a result of the attacks.

PayPal was forced to draft in 104 employees from parent company eBay to contain the damage, and shell out on hardware and software to protect itself from future actions.

MasterCard and Visa have not put a price tag on the damage caused by Anonymous during the online firestorm, but the court was told that the defendants boasted on IRC that "We have probably done some million pound of dmg to mc", which is read as a reference to MasterCard.

Three other self-confessed members of Anonymous have already pleaded guilty to the charges.