LONDON (Reuters) - Human rights lawyers began legal action against the British government on Tuesday, accusing it of involvement in the illegal transfer of a terrorism suspect from Indonesia to Egypt where they say he was tortured. Reprieve, a British-based legal rights group, says Britain knowingly allowed Mohammed Saad Iqbal Madni to be transferred from Jakarta to Egypt via a US airbase on Diego Garcia, a British-ruled island in the Indian Ocean, in 2002. Once in Egypt, Madni says he was tortured for three months and then sent to the US prison at Guantanamo Bay in Cuba where he was held for six years before being released last year without charge. He now lives in Pakistan. If successful, the case could formally link Britain to the illegal transfer of suspects across borders for the first time. Britain has admitted US rendition flights entered and left its territory but said it had been unaware of this at the time. We have made our disappointment about these flights clear with the US and secured firm new assurances that on no other occasion since September 11, 2001 has a US intelligence flight with a detainee on board passed through UK territory, the Foreign Office said in a statement on Tuesday when asked to comment on Reprieves case. Reprieve, which represents several detainees held at Guantanamo Bay, says Britain must have known about Madnis rendition via Diego Garcia because a specific chain of command was needed to authorise unorthodox flights. It says records relating to flights to Diego Garcia before 2008 have been destroyed in mysterious circumstances. It says its case is aimed at forcing the government to come clean.