European judges will rule later whether six terror suspects, including radical Muslim cleric Abu Hamza al-Masri, can be extradited from the UK to the US.
They include alleged terror fundraiser Babar Ahmad and two men accused of a role in two 1998 US embassy bombings.
The European Court of Human Rights has backed terror extraditions from the UK to the United States.
The European Court of Human Rights has considered whether human rights would be breached if the men receive lengthy sentences in certain prison conditions.
The Strasbourg court held there would be no violation of human rights for those facing life and solitary confinement in a supermax prison.
The suspects say they could be held in solitary confinement.
The court sanctioned the extradition of Abu Hamza, Babar Ahmad and three others.
They argue that they might be held in a high-security prison in Colorado, known as a "supermax" prison and claim that if convicted there is very little or no prospect of ever being released.
Judges said they would consider further the case of another suspect because of mental health issues.
The six suspects, who have been indicted on various charges of alleged terrorism in the US, say conditions of detention at a so-called "supermax" prison would amount to ill-treatment under article three of the human rights code.
The court's ruling can still hypothetically be appealed to its final Grand Chamber - but in practice very few cases are reheard in that final forum.
The European code states: "No-one shall be subjected to torture or to inhuman or degrading treatment or punishment".
The court's decision is one of its most important since 9/11 because it defines how and when the US can successfully seek the extradition of terror suspects from the UK.
Potentially significant
The men have three months to try to persuade the Grand Chamber to reopen the entire case and examine it. If the men fail to launch an appeal, they will be extradited to the United States.
European judges halted extradition proceedings concerning the suspects in July 2010, arguing that the court needed more time to consider complaints that transferring the men risked breaching their rights by exposing them to possible life imprisonment without parole and solitary confinement.
A spokesman for Babar Ahmad, who has been held for a record of nearly eight years without trial, said he would fight on against extradition.
Earlier this year, the European Court ruled that Jordanian terror suspect Abu Qatada could not be sent for trial from the UK to his homeland because evidence obtained by torture might be used against him.
Last week, he appealed in a BBC interview to be charged and tried in the UK because his alleged crimes were committed here.
Conservative MP Dominic Raab, a member of the joint parliamentary committee on human rights, said the judgement would be "very important" and that if the judges found in favour of the terror suspects it would create a "perverse situation".
The European Court said there would be no breach of human rights if the men were to be held at ADX Florence, a Federal Supermax jail used for people convicted of terrorism offences.
He added: "On the one hand our blunt extradition laws don't protect our innocent citizens from being extradited in the first place.
Abu Hamza is unlikely to be held at that jail because of his disabilities. The court also held that the life sentences each man faces would not breach human rights.
"On the other hand you'd have European human rights law protecting serious terrorist players like Abu Hamza on the basis of even if convicted fairly, they would face life imprisonment under pretty tough conditions."
But in one case, Haroon Aswat, judges said they could not yet give the go-ahead to extradition because they needed to see more submissions on his schizophrenia and how that would be treated were he sent to the US.
Michael Caplan, a solicitor who is an expert on extradition, says the ruling could have wider repercussions.
"It's obviously a very important decision because if it goes in their favour then obviously this government and the United States will have to reconsider where they are," he said.
"If it goes against the six men then obviously that will pave the way for their extradition."
The ruling is one of the most significant decisions in years from the Eurpopean Court of Human Rights because it will define how and when the UK can extradite suspects to its closest allies.
Egyptian-born Abu Hamza, who was granted British citizenship in 1986, rose to prominence when he was a preacher at Finsbury Park mosque, in north London.
He is serving a seven-year sentence in the UK for inciting racial hatred.
'Terrorist facilitator'
He is wanted in the US on 11 charges related to claims that he took 16 hostages in Yemen in 1998, promoted violent jihad in Afghanistan in 2001 and conspired to set up a jihad training camp in the state of Oregon.
US authorities have described him as a "terrorist facilitator with a global reach".
Babar Ahmad, 37, has been held without trial in a UK prison for nearly eight years and has been refused bail since his August 2004 arrest on a US extradition warrant.
The British Muslim denies terror-related charges.
In an exclusive interview with the BBC broadcast last week, he said: "I do not hold the Americans responsible for anything that has happened to me but I think it is fair to say that I am fighting for my life - and I am running out of time."
Four other men - Haroon Rashid Aswat, Talha Ahsan, Adel Abdul Bary and Khaled Al-Fawwaz - will also be subject to the ruling made by European judges.
Mr Al-Fawwaz and Mr Bary are accused of being involved in US Embassy bombings in Kenya and Tanzania in 1998.
Mr Aswat is accused of a being part of an alleged terror camp in Oregon and Mr Ahsan is accused by a US court of running an extremist website and funding the Taliban.
