The House of Lords is debating a change in the law that could save self-confessed Pentagon hacker Gary McKinnon from extradition.
The change to the Policing and Crime Bill would let judges stop an extradition if most of the crime was committed in the UK and it would serve justice to have the trial in the UK.
Home secretary Alan Johnson said earlier he had "stopped the clock" on McKinnon's extradition so that he could consider new medical evidence. McKinnon suffers from Asperger's syndrome, an autistic condition, and has suffered recurrent bouts of depression since his initial arrest in 2002.
The 2003 extradition treaty between the US and UK allows the US to ask for UK nationals to be extradited without having to provide prima facie evidence of wrong-doing. British officials must supply evidence of "probable cause" before the US will extradite one of their nationals for trial in the UK.
Parliament was quick to ratify the treaty in the wake of the 9/11 attacks, but Congress still has not. This has led to criticism that the US was abusing the agreement to pursue non-terrorist suspects, or, as in McKinnon's case, where it had little or no hard evidence beyond McKinnon's own confession that he hacked into Pentagon computers.
McKinnon denies US charges that his activities caused damages worth some $800,000, and has fought several court cases to stand trial in the UK rather than the US.
Lord Thomas of Gresford, the Liberal Democrat peer who is proposing the changes, said, "We believe there is a significant imbalance between the requirements of US citizens to this country and the extradition of British citizens to the USA," the Telegraph reported.
MPs on the home affairs select committee will hold a special hearing on the treaty on 10 November.