Former computer hacker Gary McKinnon has vowed to fight extradition to the US, where he faces decades in jail.
Gary McKinnon said he was looking for information about UFOs
He told the BBC he was lodging a High Court appeal against Home Secretary John Reid's decision to allow him to be sent to America.
Mr McKinnon, 40, from London, said he would take the case to the European Court of Human Rights if he needed to.
He is accused of hacking into US government computer networks, causing thousands of pounds worth of damage.
One American prosecutor alleged he had carried out "the biggest military computer hack of all time".
Mr McKinnon admitted that he accessed US government systems saying he was looking for material about UFOs and "suppressed technology".
But he denies being an expert hacker, saying he would be regarded as a "complete amateur" within the IT community.
If he were to be found guilty in the US, he would face millions of dollars in fines as well as a long jail term.