US authorities want to extradite a UK hacker accused of breaking into nearly 100 government and private-sector computers.

Gary McKinnon, an unemployed computer programmer living in London, is alleged to have caused damage estimated at more than £500,000. And consultants have warned that other hackers may be able to replicate the attacks.

Prosecutors claim McKinnon crippled military networks in New Jersey and Washington for days and hacked into systems run by Nasa and other private and public-sector institutions.

They allege that McKinnon, who called himself Solo, exploited known vulnerabilities in Windows NT to steal passwords, delete files, monitor traffic and shut down computer networks on military bases from Pearl Harbour to Connecticut, before being arrested by the UK National Hi-Tech Crime Unit (NHTCU).

"Nasa contacted us first asking for help in finding a UK-based hacker," Judy Prue, spokeswoman at the NHTCU, told vnunet.com.

"We started work identifying the target and when the officer in charge heard of the army and navy attack he linked the two. We identified and arrested the man and are taking no further action while extradition proceedings are continuing," she added.

US authorities want to charge McKinnon with seven counts of computer fraud, and he faces separate trials in Virginia and New Jersey. The charges carry a maximum penalty of upto five years in jail and a fine of $250,000 (£160,000).

Although hacker extradition is extremely rare, the US and the UK were among 26 countries that signed the Council of Europe convention on Cybercrime last year. The treaty provides for hacker extradition even among countries without formal agreements.

US attorney Chistopher Cristie said: "This is a grave intrusion into a vital miltary computer system at a time when we, as a nation, had to summon all of our defences against further attack."

McKinnon is claimed to have caused more than $900,000 (£567,000) worth of damage to computers in 14 states in 92 attacks between March 2001 and March 2002. After gaining entry to computer systems he used a software programme called RemotelyAnywhere to monitor network traffic and delete files.

He is alleged to have downloaded hundreds of user passwords, and in several cases destroyed critical files, making the computers unable to function.

In one incident is accused of shutting off more than 1,000 military users in the Washington area from their network for three days. He is also said to have caused $290,000 (£183,000) worth of damage to the Earle Navy Weapons Station in New Jersey.

Pentagon officials have insisted that the material seen by McKinnon was sensitive but not classified. But security consultants are unconvinced.

"[It appears as if] Gary had pretty much full, unchecked access to the networks and was able to move freely between different parts," said Matt Tomlinson, business development director at MIS Corporate Defence Solutions.

"Hackers are normally fairly open within a trusted circle of friends and there is a better than average chance that more people than just Gary had access, and know about potential backdoors and information.

"With the intimate knowledge that Gary and potentially others have to this part of the network it would be very easy for them to go back," Tomlinson said.