The European Court of Human Rights Tuesday blocked Britain from extraditing an alleged top aide of Osama bin Laden to Jordan, saying evidence against him may have been obtained through torture.
Abu Qatada, 51, a radical Britain-based Muslim cleric who has fought a six-year battle to remain in the country, faced a "flagrant denial of justice" if he was returned to Jordan, the court said in its ruling.
The Strasbourg-based court said there was a "real risk of the admission of evidence at the applicant’s retrial obtained by torture of third persons".
British Home Secretary Theresa May said she was "disappointed" with the European court's decision but added that Qatada would remain in prison while "all the legal options" are considered.
Abu Qatada, a Jordanian of Palestinian origin who is also known as Omar Mohammed Othman, was once labelled the late Al-Qaeda leader's right-hand man in Europe by a Spanish judge.
He is included on a UN list of people associated with the presumed perpetrators of the September 11, 2001 attacks in the United States.
The cleric has been convicted in his absence in Jordan of involvement in two attacks and Amman has repeatedly urged London to extradite him.
The court said however that it "finds that there is a real risk that the applicant’s retrial would amount to a flagrant denial of justice" -- a violation of Article 6 of the Convention on Human Rights.
It said Abu Qatada had "presented further concrete and compelling evidence that his co-defendants were tortured into providing the case against him.
"He has also shown that the Jordanian State Security Court has proved itself to be incapable of properly investigating allegations of torture and excluding torture evidence."
But the court also said it was confident that an agreement between Britain and Jordan that Abu Qatada would not be tortured if he was extradited would be upheld.
Abu Qatada has had political asylum in Britain since 1993 when he came to the country on a forged passport, and been in and out of prison ever since.
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Jordan says Abu Qatada conspired to carry out 1998 bombings in the Jordanian capital Amman on the American School and the Jerusalem Hotel.
He also funded a terrorist network known as Reform and Challenge (Al-Islah Wal Tahhadi) which was dismantled in 1999, but received an amnesty for those charges.
In August 2005 he was arrested in preparation for his extradition following a request by Jordan. He 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 is currently being held at Long Lartin prison in Worcestershire, central England.
May said the British government would study its options after the ruling.
"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," she said.
"In the meantime, Qatada will remain in detention in the UK."
Britain is seeking to use its presidency of the Council of Europe, which it holds until May, to push through reforms of the court, and Prime Minister David Cameron will use a speech in Strasbourg next week to set out his priorities.
"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," Cameron's spokesman told reporters.
Rights groups in Britain hailed the decision to keep Abu Qatada in Britain.
Roger Smith, director of the campaign group Justice, said the European court had sent a "groundbreaking message on the exclusion of evidence obtained after torture".
Amnesty International also welcomed the ruling but said the decision that diplomatic assurances would reduce the risk of further torture was a "setback".