A computer wizard who devised three viruses which between them attacked 27,000 computers in 42 countries must serve his full time behind bars.
Simon Vallor, 22, who devised his trio of "worms" between June and December 2000, was jailed for two years in January after pleading guilty to three counts of releasing a computer virus on the internet.
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Vallor, a web designer from Llandudno, always maintained that he did not realise the extent of damage the viruses would do, his counsel, Grant Van-Stone, told London's Court of Appeal.
Mr Justice Aikens, sitting with Mr Justice Penry-Davey, said Vallor wrote three viruses - Gokar, Redesi and Admirer - which he sent out via e-mail.
He said they were "highly disruptive" and added that the Gokar virus was the most virulent of the three and had affected companies and organisations, and was still active in December 2002.
Mr Justice Aikens said Vallor was eventually tracked down in February 2002 by the North Wales police, working with the FBI, through his computer name of "Gobo" which he used to send the e-mails.
He said the aggravating features of the case were the potential disruption to computers "on a grand scale", the fact that it was planned and was done over a period of time.
Mr Van-Stone unsuccessfully argued that Vallor's sentence should be cut because of his relative youth, previous good character and the fact he co-operated with the police.
But dismissing the appeal, Mr Justice Aikens said that Vallor's crimes were "calculated and disruptive".