In an unprecedented public intervention, head of the MI5 Jonathan Evans on Friday stepped out of the shadows to deny allegations that it had tried to cover up its involvement in the torture of a terror suspect while in American custody.

Normally, British intelligence chiefs are not known to make public statements and until recently even their names were kept secret.

Mr. Evans’s outspoken comments, dismissing the allegations as a “conspiracy theory”, came a day after the second most senior judge Lord Neuberger, the Master of the Rolls, said the MI5 had a “culture of suppression” which undermined public trust in its assurances about respecting human rights.

Lord Neuberger was part of a three-member bench of the Court of Appeal which ordered the release of hitherto secret documents relating to the mistreatment of Binyam Mohamed, a British resident of Ethiopian origin and a former Guantanamo Bay prisoner.

Mr. Mohamed (30), who was arrested in Pakistan in 2002, claims that his American captors flew him to Morocco where he was beaten, deprived of sleep and his genitals were cut with a scalpel. He also alleged that the MI5 was complicit in his torture and fought a long legal battle to demand release of documents that, he claimed, proved his allegation.

The government opposed his appeal arguing that the intelligence contained in the documents was American property and Britain had no authority to release it. It argued that Americans would stop sharing intelligence with Britain in future if the information was made public.

However, much to the government’s embarrassment, the court rejected its case earlier this week and published a summary of the relevant information showing that Mr. Mohamed’s treatment was “cruel, inhuman and degrading”.

Mr. Evans said Lord Neuberger’s comments about a “culture of suppression” in the MI5 were “the precise opposite of the truth”. Writing in The Daily Telegraph, he said allegations such as these and human rights campaigns would be used by “our enemies” to “undermine our will and ability to confront them”.

Home Secretary Alan Johnson called the allegations of a cover-up a “ludicrous lie” but MPs called for an independent inquiry.