A man who infiltrated a group with alleged links to an organisation Tony Blair wants banned, has claimed the group is radicalising young Muslims to become potential terrorists.
It is claimed cell members were shown propaganda videos about Guantanamo Bay
Hizb ut-Tahrir is banned in several Arab countries and is being scrutinised by the Home Office.
But a mole inside a cell said to be connected to the organisation, claims it makes recruits commit crimes to test their loyalty and uses brain-washing techniques to incite them to hate "non-believers".
"Jay" told a joint BBC File on 4 and Newsnight investigation that he was recruited to the cell and made to swear loyalty to Hizb ut-Tahrir.
He said its members were taught not to acknowledge each other even attending the mosque and said of the cell members: "These are people willing to beat you up and sell you drugs and make sure you die."
Jay said after one talk he was asked to prove his loyalty.
"They said Allah says you have to go and intimidate those boys across the street and get money off them."
He said a few weeks ago he was told to go to a house near Croydon, south London, where his cell leader showed a propaganda video claiming to show mistreatment of prisoners held at the US camp in Guantanamo Bay, Cuba.
"It was shocking. It showed the way they are being battered around and it was really graphic - extremely graphic.
"It was fake material but it was really good material."
He said the video provoked great anger in the group.
"One guy said: 'These non-believers, they do not deserve to treat our people this way.
"Then another guy shouted: 'I will kill them, I will kill them.'"
A statement issued by Hizb ut-Tahrir said it was a non-violent organisation which did not promote criminal activity.
"Part of our work is to persuade the youth to turn back on the life of crime," it added.
"The radicalisation of Muslims, particularly young Muslims, is the product of their anger at the government's unjust and brutal war in Iraq," the statement said.
Home Office Minister Tony McNulty said Hizb ut-Tahrir was "currently under review and if we think we need to take action it will be proscribed".
But he added: "We cannot ban an organisation on our own volition we have to operate under the rule of law."
Mr McNulty said the Home Office had to have evidence of alleged criminal activity or the glorification of terrorism.