Britain’s home secretary, Theresa May, said that she was “disappointed” by the ruling, and that she would “consider all the legal options available.” She added, “It is important to note that this ruling does not prevent us seeking to deport other foreign nationals.”

The European ruling addresses an issue that has preoccupied three British governments across a decade — what to do with resident terrorism suspects who the authorities have argued cannot be tried in open court here without revealing intelligence secrets. They often cannot be deported to home countries with a history of mistreating prisoners.

Mr. Othman, 51, initially won the right to remain in Britain in 1993, the year he sought asylum, claiming that he had been tortured in Jordan. A father of five, he has subsequently been sought by the authorities in nine countries, including the United States. Tapes of his sermons were found in the Hamburg apartment used by Mohammed Atta, one of the hijackers involved in the Sept. 11, 2001, attacks.

Speaking to the BBC after the attacks, Mr. Othman characterized them as “martyrdom” operations that should be supported by all Muslims. A 2006 report by the RAND Corporation subsequently described him as “Al Qaeda’s spiritual leader in Europe.”

Britain’s close links with the Middle East and Pakistan, its formerly generous asylum policies and a history of protecting free speech also combined to make it fertile ground for many others seeking to promulgate extremist interpretations of Islam. Former Prime Minister Tony Blair was fiercely criticized for trying to address Britain’s problems with Islamic extremism by introducing indefinite detention without trial for “suspected international terrorists” in the wake of the Sept. 11 attacks. Subsequent efforts have included more robust laws against inciting religious hatred, “control orders” that seek to monitor suspects without jailing them, tightened asylum procedures, and redoubled efforts to deport suspects.

All those measures have been subject to appeal at the European court, which sits above Britain’s own Supreme Court as a final arbiter of appeals.

Many terror suspects sought by the United States, including the preacher Abu Hamza, have argued that the threat that the United States will hold them in isolation in supermaximum security prisons or at the Guantánamo Bay detention center in Cuba, and the possibility of the death penalty, constitute human rights violations.

Aides to Mr. Cameron said that when the prime minister travels to Strasbourg next week to address the Council of Europe, which oversees the court, he will map out British proposals for limiting the court’s powers to overrule the findings of the domestic courts of European Union states. One idea that has gained support among British critics of the European court is a “democratic override” to allow national parliaments to annul court rulings on some issues, including those that affect national security.