Five judges at the country's highest court retired on Tuesday to consider a final appeal against extradition by a computer enthusiast wanted in Washington for the "biggest military hack of all time".
Gary McKinnon, 44, is accused of causing hundreds of thousands of dollars of damage after breaking into dozens of computers owned by NASA, the Pentagon and the U.S. military.
Using the online nickname "Solo", he hacked into some of the most sensitive U.S. government computer networks from his London home between February 2001 and March 2002.
One attack brought down the U.S. Army's entire Washington military district network of more than 2,000 computers for 24 hours, the five law lords sitting at the House of Lords were told during a two-day hearing.
At the time of his initial indictment, Paul McNulty, the then U.S. Attorney for the Eastern District of Virginia, said: "Mr McKinnon is charged with the biggest military computer hack of all time."
In an interview with Reuters in 2006, McKinnon said he was just a computer enthusiast who wanted to find out whether aliens and UFOs really existed.
He argues that U.S. officials have broken extradition rules by threatening him with a longer jail sentence unless he pleads guilty.
According to his legal team, one U.S. official told him the authorities in Washington wanted to see him "fry".
McKinnon says the extradition is an attempt to prosecute him for his "nationality or political opinions" and would breach his human rights and break English court rules.
His last appeal to the High Court in London failed after two senior judges ruled he should face trial in the United States.
The law lords are expected to give their ruling before the end of July.
