Abu Qatada has been involved in a long legal battle
Radical Islamic cleric Abu Qatada has been issuing messages to his followers from his British jail cell, according to anti-extremism researchers.
Statements said to be from the Jordanian have appeared recently on a number of extremist websites.
The Quilliam Foundation think tank say Abu Qatada has released three letters from prison in four months.
But the Prison Service says Quilliam's claims that it has been incompetent are "completely unfounded".
Abu Qatada was once dubbed Osama bin Laden's spiritual ambassador to Europe and recently lost a battle to stay in the UK.
He is asking the European Court of Human Rights to overturn a decision by the Law Lords to allow him to be returned to Jordan.
Abu Qatada is presently held in isolation at the high security Long Lartin Prison in Worcestershire.
But the Quilliam Foundation says statements have been appearing online under his name, circulating on both English and Arabic language websites.
In the statements, the writer using the name Qatada congratulates al-Qaeda fighters, claims that the British government opposes Islam and says Muslims should never join the police or army in a non-Muslim country.
The most recent posting appeared on a jihadist forum last month and details life inside a British maximum security prison.
There is no direct proof that the statements were issued by Abu Qatada - but Quilliam says evidence links the posts to Islamist associates of Abu Qatada. The content of the messages is also consistent with Abu Qatada's previous statements.
Ed Husain of the Quilliam Foundation says it was possible that the leaks may have occurred through Abu Qatada's visitors, despite them being restricted.
He said: "Time will tell how it's done, and that's why we're calling for an investigation by Jack Straw and others who are involved in this to get to the bottom of this, and ensure that we don't have this psychological warfare launched against Muslims and others across the world from British prisons."
Justice Minister Shahid Malik said the Quilliam Foundation's comments were "very very surprising".
He added: "Their so-called research doesn't actually have any evidence, there's no basis for these comments.
"The fact is this is alarmist, there's no evidence that's been given to us, the research seems pretty lame at best, and I've got to say this is good for the Quilliam Foundation because they're all about publicity, they've generated a lot of that.
"But it is alarmist, it is unhelpful, it is sensationalist."
A spokesman for the Prison Service discounted the accusations - saying Abu Qatada was closely monitored as a Category A high risk inmate.
"In line with security measures to manage high risk Category A prisoners, this individual is strictly monitored and has absolutely no access to prisoners other than those in the small unit in which he is held, none of whom are considered vulnerable to radicalisation," said the spokesman.
"All communications, including telephone calls, are strictly monitored and those not conducted in English are translated.
"This prisoner has absolutely no access to the internet, and cannot personally contribute to websites. However, we are unable to prevent third parties from publishing information which is, or claims to be, on other people's behalf or in their name."
Correction 9 April 2009: An earlier version of this article suggested that messages from Abu Qatada were being leaked from his British jail cell 'possibly through his lawyers'. We accept that this was not the case and unreservedly apologise to Birnberg Peirce & Partners solicitors who have represented Abu Qatada wholly professionally for a number of years.