The suspects include three Britons and the Egyptian-born radical cleric Mustafa Kamal Mustafa - also known as Abu Hamza Al-Masri, the one-eyed, hook-handed hard-liner accused of setting up a terrorist training camp in rural Oregon.
The European Court of Human Rights gave Britain until Sept. 2 to respond to questions about the punishment they will face. If convicted of charges filed between 2004 and 2006, they could get lifelong jail terms without parole in maximum security conditions, including concrete furniture, timed showers, tiny cell windows and no communications with the outside world.
Al-Masri claims he has lost his Egyptian nationality, but Britain considers him an Egyptian citizen. He has also been linked to the taking of 16 hostages in Yemen in 1998 and to preaching jihad in Afghanistan.
The other suspects are Al-Masri's alleged coconspirator, Haroon Rashid Aswat, and Babar Ahmad and Syed Talha Ahsan.
Ahmad is accused of running websites to raise money, appeal for fighters and provide equipment such as gas masks and night vision goggles for terrorists. Ahsan is charged with conspiring to support terrorists via the Internet.
After the four exhausted their appeals against extradition in British courts, they turned to the Strasbourg, France-based court.
They argued incarceration in the US will be so long and harsh it would amount to a violation of article 3 of the European Convention on Human Rights of 1950. The article says, "No one shall be subjected to torture or to inhuman or degrading treatment or punishment." The court dismissed the four suspects' argument that as non-US citizens their trial would be "a flagrant denial of justice." It also dismissed their fear that the four would be declared enemy combatants or sent to a third country - past practices that have been abandoned.
But it asked Britain to say if decades of incarceration in maximum security was not inhumane or degrading punishment.
Before making a final ruling, it also asked if the four can ever expect "transfer to a normal prison" or a reduction in sentences over time.
The court is an arm of the 47-nation Council of Europe, Europe's premier human rights watchdog. Its judgment are binding on members.
Britain is holding several convicted terrorists in high-security prisons, including in the notorious Belmarsh prison in London.
Separately, the European Parliament approved a long-awaited deal to share financial data with the US in suspected terrorist cases, after Washington agreed to major concessions to allay concerns over privacy.
The agreement was adopted Thursday with 484 votes to 109 with 12 abstentions and will take effect Aug. 1.
It allows US officials to request financial data from European banks if they suspect accounts are being used by people with terrorist links. Officials must provide European authorities with reasons for their suspicions, rectify inaccurate data and grant legal redress in US courts if information is abused.
The White House welcomed the decision, calling it an important counterterrorism tool.
"The threat of terrorism faced by the United States and the European Union continues and, with this agreement, all of our citizens will be safer," President Barack Obama said in a statement. He said the US program of tracking terrorists' funds has yielded critical leads since 2001.
