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Instead, the hackers triggered a "frenetic rather than panic-stricken" response by MoD officials as the intrusion was characteristic of an information-warfare attack, when enemies attempt to destroy or disrupt military communications networks.

The hackers are being investigated by Scotland Yard's Computer Crimes Unit and the Communications Electronics Security Group at GCHQ, with assistance from the US Air Force.

American hackers passed on information that implicates hackers in southern England. Scotland Yard is assembling evidence and arrests are expected soon. 

The hackers intercepted the link between the Skynet's control centre and the ground station. The source said the hackers "managed to reprogram a satellite control system. In many ways, the clever thing was not to lose the satellite.

Last week, Margaret Beckett, Leader of the Commons, warned of the growing risk of malicious electronic attacks on Britain's critical information infrastructure. "Hijacking a satellite is one of the first activities in an infowar attack," the source said. Defence staff examined several other classified points that would be expected to be attacked in the event of an information warfare assault. "Initially, the attack was thought to be an overt act of war. Now we think it was a mischievous act."

A spokesman for Scotland Yard said a computer hacker was being investigated. "The hacker is believed to be targeting several different international sites, some of which may include military installations," he said.

Britain has three satellites that form what is known as Skynet 4, the most modern generation of British military satellites. The first generation was launched in the 1960s and Skynet 4 went up in the late Eighties. The satellite that was infiltrated is believed to cover Scandinavia, the North Sea and northern England. Like all the MoD's satellites, and the two others Britain operates for Nato, it is controlled by the Royal Air Force.

The British hacking community was "astounded and envious" at the audacity of the attack, said one British hacker. "We guess that it is an unusual crew, probably a group of students with access to the control system," he said.

The hacker group is believed to have used a "recipe" describing how to attack satellite control command systems, published several years ago by a Briton who subsequently fled to Japan to avoid arrest for another hacking incident.

Several years ago, an American hacker called Capt Midnight grabbed control of an American television satellite. He replaced some of the channels with a test card that protested at the introduction of pay-television. 

Geoff Bains, editor of What Satellite?, said: "It has always amazed me that more people have not done this. You just have to learn a few control codes and send up your own signal to play around with a satellite yourself."