Radical cleric Abu Qatada cannot be sent back to Jordan because it would be a "flagrant denial of justice", human rights judges ruled today.
The firebrand preacher, described as "Osama bin Laden's right-hand man in Europe", won his appeal to the European Court of Human Rights against the UK's efforts to deport him with assurances that he would not be tortured.
Home Secretary Theresa May said she was "disappointed" but it was "not the end of the road" and Qatada would remain in prison while "all the legal options" are considered.
The judges ruled that sending him back to face terror charges could lead to evidence obtained by torture being used against him, denying him his right to a fair trial.
"In the absence of any assurance by Jordan that the torture evidence would not be used against Mr Othman, the court therefore concluded that his deportation to Jordan to be retried would give rise to a flagrant denial of justice," a court spokesman said.
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.
The government can now make a final attempt to appeal against the judgment before it becomes binding in three months' time.
May said: "I am disappointed that the court has made this ruling.
"This is not the end of the road, and we will now consider all the legal options available to us. In the meantime, Qatada will remain in detention in the UK."
Qatada, also known as Omar Othman, featured in hate sermons found on videos in the flat of one of the 9/11 bombers.
Law Lords ruled almost three years ago that he could be sent back to Jordan, but he appealed to the court in Strasbourg, claiming he would be tortured if sent home.
Today, the court ruled there would be a breach of his right to a fair trial "given the real risk of the admission of evidence obtained by torture at his retrial".
The court also found that, in relation to each of the two terrorist conspiracies charged against Othman, the evidence of his involvement had been obtained by torturing one of his co-defendants.
Today's court ruling went against the UK Law Lords' 2009 judgment, agreeing instead 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 because evidence against him could have been extracted through torture.
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.
Britain is seeking to use its presidency of the Council of Europe, which it holds until May, to push through reforms of the Court of Human Rights.
Prime Minister David Cameron will use a speech in Strasbourg next week to set out his priorities for change.
His official spokesman told reporters: "We have been talking for some time about reform for the European Court of Human Rights and we are using our presidency of the Council to try to make some progress on that issue.
"In particular, there is a massive backlog of cases which we want to address - something like 150,000 cases, which suggests that something isn't working quite as it should be.
"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. I think the PM will say more about that agenda next week."
Human rights group Liberty called for Qatada to be tried for terrorism in the UK "without further delay".
Shami Chakrabarti, the group's director, said: "Today's decisions from the European Court of Human Rights show considerable trust in the UK legal system but understandable concern about Jordanian law.
"The court found that torture and evidence obtained that way is widespread in that country. So it is clear that, if Abu Qatada is to be tried for terrorism, this should happen in a British court without further delay."
However, Gerard Batten, the UK Independence Party's home affairs spokesman, said: "Yet again we see how the British courts are impotent in the face of the European Court of Human Rights.
"We can no longer decide who lives in this country and the British public are getting sick and tired of seeing our judicial system trampled on like this. This ruling sends out a message that anyone can have unlimited access to the UK - be they terrorist or criminal."
Qatada, a father of five who came to Britain in 1993 on a forged passport, was released on bail in the summer of 2008 but returned to prison in November the same year over fears he would try to abscond.
He was first detained in 2002, when an immigration court described him as a "truly dangerous individual".
The Special Immigration Appeals Commission said he had given religious authority to numerous high-profile terrorists across the world, including those behind the September 11 attacks.
He was later bailed, but placed under a control order. In 2005 he was arrested in preparation for his deportation to Jordan, but was again released on appeal.
Qatada has issued a series of influential fatwas, or religious rulings, in support of the killing of non-believers and a number of his videos were found in the Hamburg flat of Mohammed Atta, one of the ringleaders of the September 11 hijackers.
In 1999, he was convicted in his absence in Jordan of conspiracy to carry out bomb attacks on two hotels in Amman and providing finance and advice for another planned series of bombings. He was sentenced to life imprisonment.
Blair Gibbs, head of crime and justice at the Policy Exchange think-tank, said: "This is a flawed ruling that 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.
"The government spent time and money securing a guarantee from Jordan that Abu Qatada would not be tortured, but now the European Court has ignored that and struck down the UK ruling solely on the grounds that some evidence possibly obtained by torture may be used against Qatada at some future trial."
But Amnesty International welcomed the ruling, but said it was "eclipsed by the court's conclusion that diplomatic assurances can, under certain circumstances, be sufficient to reduce the risk of torture".
Former home secretary David Blunkett warned that Qatada was very dangerous.
"This man has not only justified the September 11 attacks but advocated Jihad," he told BBC Radio 4's The World At One programme.
"This man, as indicated by the judiciary, is extraordinarily dangerous and we don't want him on our streets."
Sajjad Karim, Conservative spokesman on legal affairs in the European Parliament, said 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 North West England MEP said: "In short it will make people angry.
"I believe our domestic courts are perfectly capable and should be left to control our borders and make our deportation decisions. We do not condone torture and nor do our judges."