Officials say the bailed cleric has links with Osama Bin Laden, 9/11 and the Iraqi group that beheaded Briton Kenneth Bigley
Abu Qatada is accused by the Home Office of being "a significant international terrorist" and the spiritual guide of Mohamed Atta, one of the al-Qaida terrorists who piloted a jet into the World Trade Centre on 11 September 2001.
The British authorities believe he has had links with Ayman al-Zawahiri, the former leader of Egytian Islamic Jihad, who effectively became Osama bin Laden's deputy. They also say he was a significant spiritual leader to the al-Tawhid movement, whose leader was Abu Musab al-Zarqawi and whose Iraq-based terrorist group beheaded the British man Kenneth Bigley.
The UK government says Qatada has raised funds for terrorist groups, and gives religious legitimacy to suicide bombers. Based in the UK for the last 19 years, he has spent almost all of the time since 9/11 in jail without charge.
In a 2005 summation of its case for his deportation to Jordan, the Home Office wrote: "The presence of Abu Qatada in the UK poses a continuing threat to national security and a significant terrorism-related risk to the public."
It said he had engaged in conduct that "facilitates and gives encouragement to the commission, preparation and instigation of acts of terrorism".
The security case stated: "He provides advice which gives religious legitimacy to those who wish to further the aims of extreme Islamism and to engage in terrorist attacks, including suicide bombings. A number of individuals arrested or detained in connection with terrorism have acknowledged his influence upon them."
But the UK authorities accept his role as an al-Qaida organiser was limited.
"We accept that [Qatada] encouraged and raised funds for individuals to go to Afghanistan; he may not have gone further as an organiser," the Home Office said in 2005.
Qatada has said he has used his influence to curb extremist activities, and appealed for Bigley's release. He also offered to make an appeal for the release of Alan Johnson, the BBC reporter who was seized in Gaza in 2007.
Qatada has already been convicted twice in his absence by Jordan of conspiring in terror plots. He says these convictions were underpinned by evidence obtained through torture, a claim that has led the British authorities to exclude the allegations from their own case against him.
In 1999, he was sentenced in Jordon to life imprisonment with hard labour for conspiracy to carry out terror attacks, and in 2000 he was sentenced to a further 15 years' hard labour for conspiring in the "millennium plot" to attack western and Israeli targets in Jordan on New Year's Eve.
Qatada was born in 1960 near Bethlehem, which was then in Jordan. He arrived in the UK on 16 September 1993 on a forged United Arab Emirates passport. He claimed asylum on arrival for himself, his wife and his three children, and was recognised as a refugee the following year, with leave to remain for four years.
He had been living in Peshawar, Pakistan, for two years but was forced to leave and travelled via the Maldives and Singapore to London.
He said he feared he would be tortured if he returned to Jordan because the Amman government objected to his Islamist political activities and his ideological leadership of an Islamist reform group that looked to install in Jordan an Islamic government, controlled by Islamic law, and to depose the monarchy.
In the UK, he was the imam at the Four Feathers mosque, in north London, and preached at other venues in the London area, including Stowe Club and the Fatima Centre. He also disseminated advice by personal contact, telephone and letter, and online.
According to the British case against him, in October 1999 he made a speech in which "he effectively issued a fatwa authorising the killing of Jews, including Jewish children".
He told his congregation that Americans should be attacked, wherever they were; that in his view they were no better than Jews; and that there was no difference between English, Jewish and American people.
He is also accused of guiding other terror groups besides al-Qaida, including the Armed Islamic Group (GIA), the Salafist Group for Call and Combat, and Egyptian Islamic Jihad. He has provided advice to individuals such as Rachid Ramda, the former leader of the GIA in the UK, who was arrested for his involvement in the 1995 Paris Metro bombings.
After the 9/11 attacks, Qatada went into hiding. Videos of his sermons were found in Atta's flat in Hamburg, according to reports accepted as true by the British authorities.
In a sermon on 14 September 2001, he said the attacks were part of a wider battle between Christendom and Islam, and were a response to America's unjust policies. In autumn 2002, a poem attributed to Qatada appeared online praising Bin Laden and glorifying the attacks, while in a sermon he stated that it was not a sin for a Muslim to kill a non-believer for the sake of Islam.
In October 2002, he was picked up under emergency terrorism laws that allowed indefinite detention without trial, and was held in Belmarsh jail. When the House of Lords quashed that power of detention in 2005 he was held under immigration law pending deportation.
He was freed on bail for six months in 2008, under conditions that demanded he stay inside his west London home for 22 hours a day. But then his bail was revoked for unspecified reasons, and he was returned to jail.