LONDON: Two British hackers were jailed on Friday for helping to spread a computer worm which affected thousands of machines around the world including some at the U.S. Department of Defense.

Jordan Bradley, 22, and Andrew Harvey, 23, were part of an international hacking group called "TH34t Krew" which created the "TK worm", a so-called "Trojan" programme that surfaced on the Internet sometime before February 2003.

Likened to the wooden Trojan Horse of Greek mythology which concealed enemy soldiers, "Trojans" are malicious computer programmes that are disguised as harmless information and can make their way on to a computer over the Internet.

Officers from Britain's National Hi-Tech Crime Unit (NHTCU) estimated the TK worm caused millions of pounds worth of damage, infecting 18,000 computers around the world.

The programme could also take control of computers it infected. Newcastle Crown Court jailed electrician Harvey for six months and unemployed Bradley for three months.

The men, both from northern England, had admitted conspiring to "effect unauthorised modifications to the contents of computers with the intent to impair the operation of those computers".

British police said that in a separate trial held in the United States in June, 21-year-old Raymond Steigerwalt from Indiana was jailed for 21 months and ordered to pay the Department of Defense $12,000 in restitution for his role in the plot.

Detective Superintendent Mick Deats, deputy head of the NHTCU, said organised criminals were increasingly using such worms to commit crime over the Internet.

"Companies are taking the brunt of their attempts to steal money and data, but consumers are also being hit," he said. "This year, 166 companies responding to our survey into the cost of hi-tech crime said they had lost over 70 million pounds ($124.1 million) to viruses, worms or Trojans."