An activist in the radical Islamist group Hizb ut-Tahrir is reported to be working in the immigration service headquarters.
The claim will cause embarrassment for the Home Secretary, John Reid, as he prepares to announce fresh counter-terrorism measures in the Queen's Speech today. The Home Office said last night it was urgently studying the claims on BBC2's Newsnight.
The man was claimed to be working at Lunar House, Croydon, the headquarters of the Immigration and Nationality Directorate (IND).
Hizb ut-Tahrir, which backs the introduction of sharia law, has been banned in several countries, including Denmark, France, Germany, the Netherlands and Turkey. Tony Blair said in August last year that he too wanted to see the group outlawed, but the Government decided against such a move after warnings from police that a ban would force the organisation underground.
The group insists it opposes violence and has condemned terrorist bombings in London, Madrid and Bali.
Patrick Mercer, the Conservative homeland security spokesman, said: "This shows just how hopeless the Government's counter-terrorism strategy is."Reuse content