THERESA May, the UK home secretary, is due to decide soon whether Gary McKinnon, an Asperger's syndrome sufferer, should be extradited to the US to face computer hacking charges. Last week, Jimmy Wales, the founder of Wikipedia, called on May to halt the extradition of Richard O'Dwyer, a British student facing US allegations of copyright infringement over a website.

Many in Britain believe the 2003 US-UK extradition treaty is unfair. As opposition leader, David Cameron called McKinnon "a vulnerable young man" and said: "This case raises serious questions about the workings of the Extradition Act, which should be reviewed."

The government did order a review, from Sir Scott Baker, a former judge. He concluded there was nothing unbalanced about the UK-US treaty. Under the treaty, when either side wants someone extradited it has to show sufficient grounds for an arrest. In the US there has to be "probable cause". In the UK, arrests can take place on the basis of "reasonable suspicion". The review said there were no significant differences between these two tests. Many disagree.

Critics want the UK to go back to the old system of demanding that the US make a prima facie case - evidence sufficient, if it is not rebutted, to secure a conviction. Opponents of the extradition treaty also want a "forum bar" - a rule that if a significant part of the crime took place in the UK, that is where the trial should take place. The treaty's opponents are right: being taken to a foreign land to face criminal charges is so wrenching that the requesting country should be forced to provide significant evidence why the trial should not take place in the UK.

In McKinnon's case, the courts have already said there was clearly a prima facie case against him and that the US was the best place for a trial because that is where the alleged computer damage occurred.

The reasons why, in my view, his extradition would be unconscionable are his fragile state and the extraordinary pressure US prosecutors put on him. His lawyer testified she was told by the prosecutors if McKinnon did not contest extradition and pleaded guilty, he could end up serving 18 months to two years, with about half in a UK prison. Otherwise, he could expect eight to 10 years in a US high-security jail.

The Supreme Court of Canada refused a US extradition request in 2001 after the US judge threatened those resisting it with "the absolute maximum jail sentence the law permits" and a US prosecutor said they faced being in a prison where "you are going to be the boyfriend of a very bad man".

Those who plead guilty in the UK can expect a one-third reduction in their sentences, but it is the staggering US harshness towards those who contest their charges that really underlies British opposition to extradition. UK attorney-general Dominic Grieve told a parliamentary committee that the British public lacked confidence in the US criminal justice system. No amount of treaty amending will change that.

© 2012 The Financial Times Limited