Gary McKinnon wants to be tried in the UK rather than the US
Human rights arguments against extraditing a British man accused of hacking into US military networks were not "confronted," a court has heard.
Gary McKinnon, 43, from Wood Green, London, wants to overturn a refusal to put him on trial in the UK on charges of computer misuse.
Edward Fitzgerald QC accused the Director of Public Prosecutions Keir Starmer QC of misapplying the law.
Mr McKinnon, who has Asperger's Syndrome, faces trial in America.
Mr Starmer decided there was "insufficient evidence" to support a UK prosecution under the Computer Misuse Act.
If there is no UK prosecution, Mr McKinnon would inevitably be extradited to stand trial in the US, the judges heard.
Lawyers for the DPP are arguing the decision not to prosecute was "entirely rational" and was "manifestly not one which is susceptible to judicial review".
Mr McKinnon, who was arrested by British police in 2002, has already appealed unsuccessfully to the House of Lords and the European Court of Human Rights to avoid extradition.
Mr McKinnon is accused of hacking into 97 government computers belonging to organisations including the US Navy and Nasa during 2001 and 2002.
The US government says this caused damage costing $800,000 (£500,000) at a time of heightened security in the wake of the 11 September 2001 attacks.
He claims he was looking for details of UFOs.
Mr Fitzgerald told two High Court judges in London that extraditing Mr McKinnon would lead to "disastrous consequences" because of his medical condition, including possible psychosis and suicide.
Mr Fitzgerald also said Mr Starmer had failed to confront the new evidence concerning Mr McKinnon's medical condition and deal with the human rights issues it raised.
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If sent to the US, Mr McKinnon was likely to receive a substantial prison sentence and was unlikely to be repatriated to serve his sentence, Mr Fitzgerald said.
And the process of extradition, trial and sentence would expose Mr McKinnon to "an avoidable and unnecessary risk of serious psychological suffering" with "all of the attendant disastrous consequences," he added.
This application for judicial review at the High Court in London is the second recent legal challenge in Mr McKinnon's case.
In the first, Mr Fitzgerald accused the home secretary of reaching a "flawed" decision, following medical evidence of the severe mental suffering that extradition would cause.
The judges are expected to give their ruling in both legal challenges later this month.