Christopher Weatherhead, a 22-year old Northampton University student, was convicted in December on one count of conspiracy to impair the operation of computers, contrary to section 1 (1) of the Criminal Law Act 1977.

Weatherhead - who used the online handle Nerdo - was described as being a "high-level operator" within Anonymous, playing a major role in a string of DDoS attacks.

Having pleaded guilty, another Anonymous member, Ashley Rhodes, was jailed for seven months. A third defendant, Peter Gibson, received a six month suspended term and a fourth, Jake Birchall, will be sentenced later.

Initially the group targeted music industry-related sites over anti-piracy measures but later moved on to financial services firms, including PayPal, MasterCard and Visa, in 'Operation Payback' for the blockade of payments to the whistleblowing Web site WikiLeaks in late 2010.

During the trial, it was revealed that the assault cost PayPal around £3.5 million as it scrambled to bring in extra staff and new hardware to keep its operation running.

Detective Chief Inspector Terry Wilson, Police Central e-Crime Unit, says: "Perpetrators of distributed denial of service attacks laud them as civil protests but they can be incredibly damaging to the finances and reputations of online businesses. Simultaneously, they impact on the general public's ability to use online services."