The activist, named as Abid Javaid, is said to be an official at the Immigration and Nationality Directorate in Croydon, one of the department’s most sensitive branches.

Shortly after the July 7 bombing attacks, the Prime Minister included the group in a list of those he planned to proscribe, but it has not been among those banned.

An investigation by the BBC Two programme Newsnight also claimed that the group preached hatred to young men using staged videos of persecution of Muslims. Newsnight said that Hizb ut-Tahrir targeted disaffected youngsters, particularly the unemployed and members of gangs in South London, and encouraged them to attack non-believers — a claim denied by the group’s spokesman, Abdul Wahid, on the programme.

Newsnight also highlighted the undercover work of Vigil, a monitoring group campaigning against religious extremism. Glen Jenvey told the programme that he had hacked into extremist websites glorifying violence and organising demonstrations.

The Home Office refused to confirm whether Mr Javaid worked at the Immigration and Nationality Directorate but added: “All Home Office civil servants are expected to abide by Home Office rules governing their conduct and are subject to the Civil Service Code.”

Dr Wahid told Newsnight that allegations of violent extremism levelled against the organisation were “far off the radar of what’s the truth”.

He added: “Our organisation has nothing to do with criminality, it has nothing to do with community tensions.”

David Davis, the Shadow Home Secretary, said that last night’s claims were “extraordinary”. He said: “This is a shocking revelation given Tony Blair’s insistence that this group should be proscribed and coming after illegal immigrant workers were discovered working in the Home Office.

“It is even more extraordinary following [the Home Secretary] John Reid’s assertion that his system for screening employees who work in the Home Office deserves an accolade for its efficiency. If this individual has access to sensitive immigration details, can ministers still claim they can maintain the integrity of their ID card database?”

Last night on its website, the group said it would be taking legal action against the BBC over the claims made on Newsnight and those on the BBC Radio 4 PM programme and File on 4.

The website, which refers to Hizb ut-Tahrir as the “Liberation Party”, stated: “The radicalisation of Muslims, particularly young Muslims, is the product of their anger at the Government’s unjust and brutal war in Iraq.

“To suggest that Muslim organisations such as ours, rather than the Government’s policies, have created anger within the Muslim community is disingenuous and irresponsible. It is organisations like ours that channel that anger into non-violent political activism in an attempt to repair the damage caused by government policies.”

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