Human rights judges will rule later on whether to allow the Government to extradite radical preacher Abu Hamza to the United States to face terrorist charges.
A final decision on six extradition cases is expected.
The verdict will effectively pass judgment on whether the US treatment of terrorist suspects could amount to "inhuman or degrading treatment or punishment" in breach of the European human rights code.
Hamza is serving a seven-year sentence in Britain for soliciting to murder and inciting racial hatred.
He has become the focus of growing concern over human rights rulings from Strasbourg which ministers claim could compromise national security.
The European Court of Human Rights halted extradition proceedings in July 2010, arguing it needed more time to consider complaints that transferring Hamza and others wanted in the US risked breaching their rights by exposing them to possible life imprisonment without parole and solitary confinement.
Egyptian-born Hamza was granted British citizenship in 1986.
He is wanted in the US on 11 charges related to the taking of 16 hostages in Yemen in 1998, promoting violent jihad in Afghanistan in 2001 and conspiring to set up a jihad training camp in Oregon.
US authorities have described him as a "terrorist facilitator with a global reach".
But, before the extradition request could be dealt with, Hamza had been convicted in 2006 of terrorist-related charges in Britain.
Among those facing extradition alongside him is Babar Ahmad , a 36-year-old computer expert who has been in a UK prison without trial for nearly eight years.
He has been refused bail since his arrest in August 2004 on a US extradition warrant.
Both appealed separately to the European Court of Human Rights on the grounds that their treatment and potential punishment could violate Human Rights Convention Article 3 banning torture and "inhuman or degrading treatment".
The fate of four other British nationals wanted in the US - Haroon Rashid Aswat, Seyla Talha Ahsan, Adel Abdul Bary and Khaled al Fawwaz - is also being decided by the human rights court.
All six have been indicted on various charges of alleged terrorism in the US.
In July 2010, the human rights judges said they needed more time to consider whether the eighth amendment to the US constitution - a ban on "cruel and unusual punishment" - gives the equivalent protection to article 3 of the European code.
This states: "No-one shall be subjected to torture or to inhuman or degrading treatment or punishment."
Babar Ahmad and Ahsan are accused in America of providing support to terrorists and conspiracy to kill, maim or injure people and damage property in a foreign country.
Aswat is accused of conspiring with Hamza to establish a jihad training camp in Oregon.
In January, the UK was refused permission to deport Abu Qatada to Jordan , where he is wanted on terrorist charges, without assurances that evidence gained through torture would not be used against him at his trial.
The result caused Government fury and has still not been resolved.
Conservative MP Patrick Mercer warned that the UK's special relationship with America could be affected if the extradition is turned down and said Britain would appeal if that did happen.
"America wants these people, America wants them to face justice inside that country - here's Britain, allegedly her closest ally, blocking that," he said.
Mr Mercer expressed hope that the Government would follow France and Italy and reject the court's decision if it goes against the extradition.
Fellow Tory Dominic Raab added that Britain faced the "worst of all worlds" if the court rules out extradition.
"On the one hand, our blunt extradition rules which don't protect the innocent from being extradited in the first place. On the other hand, you've got European human rights law protecting serious terrorist players like Abu Hamza on the basis that, even if convicted fairly, they would face life imprisonment under pretty tough conditions. To my mind, that's the worst of all worlds.
"What they would effectively be ruling out is terrorist extradition to the US because, frankly, of conditions of punishment I think most people in this country would say three cheers to.
"It's not on the basis they would not have a fair trial, it's on the basis it would be tantamount to torture or inhumane and degrading treatment if they were convicted and sentenced to face life in prison without parole in supermax facilities. I find it extraordinary."
