As Theresa May prepares to decide on Gary McKinnon case, review ordered by Nick Clegg says extradition terms favour US

Theresa May, the home secretary, is under pressure to block the extradition of Gary McKinnon, who is wanted in the US over computer hacking charges, after a review of the treaty between the UK and US that was ordered by the deputy prime minister had urged that it be scrapped.

The review, commissioned by Nick Clegg and undertaken by the former Liberal Democrat leader Sir Menzies Campbell, insists that the current extradition treaty overly favours America.

The finding directly contradicts the Scott Baker report ordered by May last year, which concluded that the Extradition Act was not biased, even though nine times as many Britons have been extradited as Americans.

May is yet to formally respond to that report, but will soon be making a decision on the fate of McKinnon. The Liberal Democrat intervention will be seen by some as an attempt to guide her hand in a case which has caught the public imagination.

McKinnon, who admits hacking US military computers but says he was looking for evidence of UFOs, could face up to 60 years in jail if he is successfully extradited and prosecuted.

Clegg was a staunch supporter of McKinnon while in opposition, although he has since been criticised by the 46-year-old's mother for failing to help her once he was in government.

Campbell's review, however, makes it clear that the extradition treaty enabling McKinnon's removal should be reformed by the government in the interests of public confidence. At the moment, "probable cause" has to be shown before a UK request for extradition of an American citizen will be granted. The reciprocal test for Britons is "reasonable suspicion".

The Scott Baker review concluded that there was no practical difference, but, in an online article for the Observer, Campbell, who is a QC, says: "With that conclusion I respectfully disagree. To put the matter as simply as I can, one may have a 'suspicion' that someone has committed a crime, but that is a different and lower standard than being satisfied that it is 'probable' that a crime was committed by that person.

"The proper course should be to raise the British standard to the American one, so that UK citizens do not suffer a disadvantage compared to their US equivalents."

He adds: "If the present British government is to fulfil its duty to protect the rights of its citizens at home and abroad and at the same time meet its treaty obligations, such a change is both necessary and possible, not least to restore public confidence in Britain in the process of extradition between the United States and the United Kingdom."

May must make her decision on the McKinnon case by 16 October. In July, his mother pleaded outside court for the home secretary to "show a little bit of compassion" and rule sooner, but the appeal was rejected.

If May allows the extradition to go ahead, McKinnon's lawyers are expected to apply for a judicial review to challenge that decision.

Campbell's review of the extradition treaty also argues that the UK courts should in cases similar to McKinnon's take into account the suspect's likely treatment in America and make a judgment on whether it would constitute a contravention of any human rights.

Campbell has presented Clegg with his review and it is likely to be discussed publicly at Liberal Democrat conference next week. May is due to set out the government's thinking on the extradition treaty by the end of the year.