Mr Caffrey claims a hacker's code was placed on his computer

Aaron Caffrey, 19, told a jury at Southwark Crown Court that as soon as he realised the blueprint - known as an attack script - could disable any computer system it came into contact with, he removed it from his computer.

Mr Caffrey, of Fairlane, Shaftesbury, in Dorset, is accused of hacking into the computer server at the Port of Houston in Texas.

The teenager, who suffers from Aspergers' Syndrome, a form of autism, denies one charge of unauthorised modification of computer material contrary to the Computer Misuse Act 1991.

Vessels 'at risk'

Defence barrister Ian Ross told the court that the attack script had contained the words "coded by Aaron".

But Mr Caffrey, who studied A-level computing after leaving school at 14, denied he had written the script and said he knew lots of people called Aaron in the chatrooms he visited.

He said: "Mainly I noted it was my name but that was about the extent of it."

Mr Caffrey was arrested and questioned by police in January 2002 after American authorities traced the source of the attack to a computer at his home.

Earlier, the court heard how the scheduling system at the port - the eighth largest in the world - was bombarded with thousands of electronic messages on 20 September 2001.

The attack left the port's web service, which contained vital data, inaccessible, placing vessels at risk.