A Muslim organisation which the Government chose not to ban is preaching hatred and has infiltrated the Home Office, an investigation has revealed.

Hizb ut-Tahrir is showing inflammatory videos to small groups of followers then encouraging them to attack non-believers.

An HuT member has gained a job as an information technology worker in the Immigration and Nationality Directorate of the Home Office and been given a grant to organise an event for the radical group, it has been claimed in an undercover investigation by Vigil, a group campaigning against religious extremism.

In a mocked-up video shown to new recruits, a woman interrogator at Guantanamo Bay was shown wiping blood over the face of a prisoner to make him confess. A student in his twenties, who posed as a recruit to HuT, said: "The reaction was shocking.

"The group were clenching their fists and shouting, 'We'll kill her, how can you do this to our brothers? F****** kuffars [non-believers]."

"Afterwards the mushrif [leader] asked for our opinions and people seemed to think the British were the same, they were saying, 'A kuffar is a kuffar and f****** British troops, we have to do something.'

"They said, 'we have to attack from the inside' and I think they meant terror attacks."

The young man, who is known as Jay - not his real name – was forced to rob three people to show his loyalty to the group.

The group, which met at West Croydon Mosque, comprised five black and Asian men, aged between 18 and 28, most of them unemployed and four of them converts. Each group had a number and theirs was in the fifties. "They were thugs," said Jay, who spent six months with them.

Jay said the group were shown DVDs on three occasions, each accompanied by a talk by the mushrif...