A terrorist accused of posing a grave threat to Britain's national security cannot be sent back home because it would be a "flagrant denial of justice", human rights judges ruled today.
Radical cleric Abu Qatada, described as "Osama bin Laden's right-hand man in Europe", won his appeal to the European Court of Human Rights (ECHR) against the UK's efforts to deport him to Jordan with assurances that he would not be tortured.
But Home Secretary Theresa May vowed it was "not the end of the road", saying Qatada would be kept behind bars while she considered all legal options to send him back.
These include an appeal to the court's Grand Chamber within three months or seeking further assurances from Jordan that evidence gained through torture would not be used in any trial.
The judges ruled that sending Qatada back to face terror charges without those assurances could lead to evidence obtained by torture being used against him, denying him his right to a fair trial in a "flagrant denial of justice".
But former home secretary David Blunkett warned that Qatada was "extraordinarily dangerous and we don't want him on our streets".
"This man has not only justified the September 11 attacks but advocated Jihad," he told BBC Radio 4's The World At One programme.
"I do hope it would be possible to reach an agreement with the Jordanian government that they would bring forward alternative evidence to that obtained originally, allegedly, by torture."
The Henry Jackson Society think-tank added that the court's ruling "undermines national security".
Robin Simcox, a research fellow, said: "This is a man who aspiring terrorists and jihadists from around the world sought out for spiritual advice.
"His continued presence in the UK poses a grave national security threat."
It is the first time that the Strasbourg-based court has found that an expulsion would be in violation of Article 6 of the European Convention on Human Rights, the right to a fair trial, which is enshrined in UK law under the Human Rights Act.
Blair Gibbs, head of crime and justice at the Policy Exchange think-tank, said the "flawed" ruling "drastically raises the bar for deportation cases".
"It is possible that Article 6 could now be used to stop deportation to any country that fails to provide British standards of justice - which is most of the developing world," he said.
Sajjad Karim, Conservative spokesman on legal affairs in the European Parliament, added it was "another case where the ECHR will be seen to be putting the welfare of undesirables ahead of the security and wellbeing of society at large".
The Prime Minister's official spokesman said Britain wanted to use its presidency of the Council of Europe, which it holds until May, to push through reforms of the human rights court which made the ruling.
"One of the most important issues is to make sure that the court focuses on the cases it should, instead of being a court of appeal for national judgments," he said.
David Cameron is expected to say more about reform of the court when he sets out his priorities for change in a speech in Strasbourg next week.
Qatada, also known as Omar Othman, featured in hate sermons found on videos in the flat of one of the September 11 bombers.
Since 2001, when fears of the domestic terror threat rose in the aftermath of the attacks, he has challenged, and ultimately thwarted, every attempt by the Government to detain and deport him.
Law Lords ruled almost three years ago that he could be sent back to Jordan and Lord Phillips, now president of the Supreme Court - the highest court in the land - said torture in another country does not require the UK "to retain in this country, to the detriment of national security, a terrorist suspect".
But the human rights court went against that judgment today, agreeing with the earlier 2008 decision of the Court of Appeal which said there were reasonable grounds for believing he would be denied a fair trial in Jordan.
It means the 51-year-old cleric must stay in Britain, where he is currently held at Long Lartin high-security prison in Worcestershire, for now at least.
Roger Smith, director of the campaign group Justice, said the court had sent "a groundbreaking message on the exclusion of evidence obtained after torture".
But Amnesty International warned the ruling was "eclipsed by the court's conclusion that diplomatic assurances can, under certain circumstances, be sufficient to reduce the risk of torture".
Julia Hall, an expert on human rights and counter-terrorism with the human rights group, said it was "an alarming setback for human rights".
Qatada, a father-of-five, claimed asylum when he arrived in Britain in September 1993 on a forged passport and and was convicted in his absence in Jordan of being involved in two terrorist conspiracies in 1999.
He was first detained in 2002, when an immigration court described him as a "truly dangerous individual", and has issued a series of influential fatwas, or religious rulings, in support of the killing of non-believers.
A number of his videos were also found in the Hamburg flat of Mohammed Atta, one of the ringleaders of the September 11 hijackers.
Gareth Peirce, Qatada's solicitor, said it has "always been astonishing that this country has fought long and hard" to deport her client to Jordan.
"What message would it have sent to the rest of the world if the European Court of Human Rights had held that it was acceptable to send a civilian to be tried in a military court on evidence emanating from torture?" she said.
"The court's judgment of today is long, thoughtful and complex, and sets out important guidelines for Europe's member states on a number of difficult issues.
"It would be indeed be disappointing if the implications of this judgment were not carefully and adequately digested and the United Kingdom were to continue a challenge which flies so directly in the face of internationally accepted standards."