It now seems very likely that Gary McKinnon, the north London UFO enthusiast and conspiracy theorist, will be extradited to the United States to face charges that he hacked into Nasa and military computers. The Law Lords ruled on 30 July that earlier American attempts to press a plea bargain from their suspect had been legal, leaving his defence team with no further recourse to the UK courts.

Lawyers are preparing a last-ditch appeal to the European Court of Human Rights in Strasbourg on the grounds that their client's curiosity and mischief could attract a disproportionate punishment. Few commentators rate their chances of success highly, however, after the Law Lords argued in their judgment that McKinnon would face a similar fate if tried in the UK.

The McKinnon saga has served to highlight again the troubling impact on justice of the post-9/11 extradition treaty between the UK and the United States. Signed in March 2003 by the then home secretary, David Blunkett, it removed the obligation on US law enforcement to present UK courts with prima facie evidence of criminality. Blunkett said at the time: "This new treaty will mean much closer co-operation and cut out much of the paperwork which has led to unnecessary delays in the current system and allowed criminals to exploit loopholes and deliberately thwart justice."

The treaty quickly became law without parliamentary debate, thanks to the Royal Prerogative. The "paperwork" - or evidence - formerly required to initiate extradition proceedings to the US is now replaced with a call for "written information" relating to alleged wrongdoing. That information is, in effect, assumed to be true fact.

Brian Howes, 44, and his wife, Kerry-Ann, 30, spent 214 days without charge in Edinburgh's Saughton Prison last year as a result of that assumption. Their detention lasted more than five times longer than the hotly disputed 42-day limit for terror suspects that squeaked through the Commons in June. Federal prosecutors at the Drug Enforcement Agency in Arizona are aiming to secure their extradition for allegedly supplying chemicals to crystal meth labs. The highly addictive home-made drug has ravaged poor communities across the United States in the past two decades.

The couple, from Bo'ness near Falkirk, were eventually released on bail to care for their four children after Brian Howes spent 30 days on hunger strike. They are now just weeks away from a final high court appeal that could see them packed off to the notorious federal penitentiaries of the Arizona Desert to await trial. Their last hope in the UK is to overturn the June decision by the Scottish justice minister Kenny MacAskill to approve their extradition. A conviction for unlawful importation of regulated chemicals carries a maximum sentence of 20 years' imprisonment and a fine of up to $250,000. The Howeses both face several indictments.

While Gary McKinnon freely admits he hacked US government systems, the Howeses strongly deny they broke the law by selling iodine and red phosphorus. Both have myriad innocent uses and are perfectly legal in the UK, but are strictly controlled by the US administration as part of its international crusade against drugs. Unusually, the investigation was sparked when suspicious Cleveland police contacted US counterparts during a separate, aborted firearms investigation.

Iodine and red phosphorus were two of about 60 substances the Howeses' business offered for sale on the web, but it is alleged they supplied iodine and red phosphorus knowing that the chemicals would be used to make crystal meth. The couple vehemently dispute that claim, insisting red phosphorus is often bought by amateur pyrotechnics devotees and that the iodine they supplied was clearly marked for medical use. But because of the transatlantic extradition deal, no UK court will ever hear those arguments.

The Howeses have been broken by the battle. A month without food caused lasting damage to Brian Howes's speech and memory; Kerry-Ann has been diagnosed with chronic post-traumatic stress disorder. Howes fears conditions in Arizona's jails, combined with a lengthy wait for the case to be heard, could prove devastating and has several times attempted to strike a plea bargain deal in exchange for his wife's freedom. Prosecutors have rejected his approaches.

This leaves the Howeses' inexperienced and under-resourced legal team reduced to battling Washington on points of law. If they are extradited to Arizona, the couple could face a three-year wait for a judge to hear the arguments of their public defender, who has been denied access to important documents detailing the investigation.

Our extradition treaty with Washington is not reciprocal. United States citizens suspected of crimes by UK law enforcement have not lost their right to challenge the evidence presented against them in a home court. The US constitution's "probable cause" provision meant that not even John Ashcroft, the bête noire of civil liberties advocates, who as attorney general signed the treaty on behalf of the Bush administration, could have torn down this safeguard.

Meanwhile, at the thin end of the deal, the UK authorities must show American courts "such information as would provide a reasonable basis to believe that the person sought committed the offence" - that is, evidence.