Home Secretary Theresa May has blocked the extradition of computer hacker Gary McKinnon to the US.
In a shock announcement, which is set to spark anger in the US, Ms May told the House of Commons that she was stopping his extradition on human rights grounds after medical reports showed the 46-year-old was very likely to try to kill himself if extradited.
McKinnon was accused by US prosecutors of "the biggest military computer hack of all time", but he claims he was simply looking for evidence of UFOs.
Mrs May said the sole issue she was considering was whether "extradition to the United States would breach his human rights".
There was no doubt McKinnon is "seriously ill" and the extradition warrant against him should be withdrawn, she said.
It is now for the director of public prosecutions Keir Starmer QC to decide whether he should face trial in the UK, Mrs May added.
A so-called forum bar will also be introduced to extradition proceedings to enable a UK court to decide whether a person should stand trial in the UK or abroad, Mrs May added.
It will be specifically designed to ensure it does not fall foul of "delays and satellite litigation", Mrs May told MPs.
Home Office medical evidence showed McKinnon was very likely to try to kill himself if extradited to the US, where he faced up to 60 years in prison if convicted.
The US stance also appeared to soften this summer, with government adviser John Arquilla saying they should be recruiting elite computer hackers to launch cyber-attacks against terrorists instead of prosecuting them.
Both Prime Minister David Cameron, who has held talks on the case with President Barack Obama, and his deputy Nick Clegg have previously publicly condemned plans to send McKinnon to the US.
McKinnon's mother Janis Sharp said yesterday: "People like this would not use Gary's case as a key part of an election campaign and then leave him for two-and-a-half years and then throw him to the wolves.
"It would seriously damage their reputations in terms of honesty and integrity."
McKinnon, from Wood Green, north London, was arrested in 2002, and then again in 2005, before an order for his extradition was made in July 2006 under the 2003 Extradition Act.
That triggered three successive applications for judicial review and questions about the fairness of the UK-US extradition treaty, which critics claim is one-sided.
Gary McKinnon's fight against extradition was one of a series of high-profile cases which show the need for measures to ensure trials can be held in the UK, campaigners have said.
Among the first defendants to fight against extradition to the US under the Extradition Act 2003 were the so-called NatWest Three.
Bankers Gary Mulgrew, Giles Darby and David Bermingham fought and lost a four-year battle against extradition to the US over allegations of conspiring with former Enron executives to dupe the bank out of 20 million US dollars (£12.4 million).
The men later admitted one charge of wire fraud and were sentenced to 37 months in jail.
But their case prompted an emergency debate in the Commons as MPs and supporters argued that their alleged offences were committed in the UK so they should be tried here instead.
The row prompted Nick Clegg, then the Liberal Democrat home affairs spokesman but now Deputy Prime Minister, to call for the Government to change the "lopsided" treaty.
The British people do not understand "why there appears to be such an imbalance between the minimal information required to extradite a UK citizen to the US compared to the more substantive justification required to extradite US citizens to the UK", he said.
"This is an issue of overwhelming public interest, yet it has been a real struggle to get the Government to acknowledge the significance of this issue."
Retired businessman Christopher Tappin, from Orpington, Kent, was extradited to the US in February on charges of conspiring to sell parts for Iranian missiles.
The 65-year-old former president of the Kent Golf Union says that for justice to be done he should have been tried by a jury of his peers in the UK, not a jury 3,000 miles away who do not share a common cultural background.
He denies attempting to sell batteries for surface-to-air missiles which were to be shipped from the US to Tehran via the Netherlands and goes on trial next month. He faces up to 35 years in jail if convicted.
And some 250,000 people have signed a petition launched by the founder of internet encyclopaedia Wikipedia to block the extradition of British student Richard O'Dwyer to the US on copyright charges over alleged offences which took place in the UK.
Jimmy Wales, 46, came out in support of the 24-year-old, who faces up to 10 years in prison if convicted of charges relating to his website, TVShack.net.
"America is trying to prosecute a UK citizen for an alleged crime which took place on UK soil," the petition states.
O'Dwyer's supporters argue that as the site, which linked to other sites that streamed pirated television programmes, did not host material itself he should not face any charges and should therefore not be extradited.