Melanie Riley of the campaign group Friends-Extradited added that the case should prompt a complete overhaul of the extradition arrangements between Britain and the US.
She said: "Britain was sold a pup in 2003 - it's time for this Government to right the wrongs.
"Gary's 10 years spent living on a knife-edge has been nothing short of cruel and unnecessary punishment.
"The impact of extradition on the defendant, their friends and families is so severe that extradition should be the last, not first, resort in Britain's efforts to fight crime."
Mr McKinnon, from north London, was a talented computer enthusiast who has said he was only interested in proving that the US is covering up information about alien technology.
But he was accused of carrying out the "biggest military computer hack of all time" by breaking into US Army, Navy, Air Force, Nasa and Department of Defence systems in 2001 and 2002.
He was first questioned by British police in 2002 and extradition proceedings against him began in 2005, prompting appeals all the way to the European Court of Human Rights.
After the election the new Home Secretary agreed to look again at the medical evidence, and after a series of further delays she will announce the final decision on Mr McKinnon's future on Tuesday. At the same time Mrs May will publish her long-awaited response to a review of the extradition treaty with the US.