Hearings in private at terrorism trial of Rangzieb Ahmed were not intended to conceal evidence of official wrongdoing, says CPS.

The Crown Prosecution Service has defended its role in a series of secret court hearings during which evidence of British involvement in the torture of a terrorism suspect in Pakistan was heard behind closed doors, with the public and media excluded.

In an operation later denounced in the Commons as an "obvious case of the outsourcing of torture", MI5 and MI6 officers and detectives from Greater Manchester police all played a part in the events that led to Rangzieb Ahmed being unlawfully detained in Pakistan, where three of his fingernails were ripped out.

But their involvement was largely concealed from the public as a result of the CPS's successful application for the use of in camera procedure, covering much of the legal argument that preceded Ahmed's trial on terrorism charges at Manchester crown court.

The CPS has denied that its use of such procedure was intended to conceal evidence of official wrongdoing, saying it was "legally permissible" and was always "proper and correct".

In emails to the Guardian, the CPS also denied that its conduct during the case would have any bearing upon its work in the Binyam Mohamed case, in which it is being consulted by Scotland Yard detectives investigating MI5's role in his alleged torture in Pakistan and Morocco.

However, the CPS failed to explain why it had applied for in camera hearings in the Ahmed case, other than to say that they had been authorised and controlled by an experienced high court judge. It also declined to comment on subsequent reports by a United Nations special rapporteur on human rights and parliament's human rights committee, which both concluded that conduct of the sort that was disclosed during the Ahmed case amounted to official complicity in torture.

Last month Human Rights Watch reported that there was clear UK complicity in the torture of Ahmed and several other British citizens detained in Pakistan. In a report entitled Cruel Britannia: British Complicity in the Torture and Ill-treatment of Terror Suspects in Pakistan, the New York-based group condemned the British government for actions that it said were cruel, counterproductive and in clear breach of international law, and concluded that its conduct in the Ahmed case, and others, had put it in a "legally, morally and politically invidious position".

Ahmed, 34, from Rochdale, Greater Manchester, was jailed for life at the end of his trial after being convicted of being a member of al-Qaida and directing a terrorist organisation. He also admitted membership of a banned Kashmiri militant organisation.

Much of the evidence on which he was prosecuted was gathered while police and MI5 kept him under surveillance in Manchester and Dubai during 2005. In January 2006, when Ahmed made plans to fly to Pakistan, police decided not to arrest him. Nor did they ask the CPS whether they had gathered sufficient evidence to charge him. Instead, as the Commons later heard, MI6 contacted the Pakistani intelligence agency the Inter-Services Intelligence Directorate (ISI), whose use of torture has been widely documented. MI6 warned the ISI that Ahmed was a dangerous terrorist and suggested it might wish to detain him. MI5 officers and Manchester detectives drew up a list of questions for the ISI to put to Ahmed. By the time he was deported to the UK in September 2007 three fingernails were missing from his left hand. Ahmed said he was also severely beaten, whipped, threatened and deprived of sleep for long periods.

The fact that MI5 and Greater Manchester police drew up questions that were handed to the ISI emerged in open court. When the Guardian reported this, a CPS lawyer threatened to have the journalist responsible arrested. Asked why one of its lawyers would threaten the arrest of a journalist for reporting what was said in open court, the CPS said the lawyer could not recall.

Reporting on the other details of the operation, which were heard in secret, would have been an offence under the Contempt of Court Act, however, until David Davis, the former shadow home secretary, made use of parliamentary privilege to disclose them in the Commons.

The trial judge ruled that the UK authorities had not outsourced Ahmed's torture, although the version of his ruling that is open to the public does not dismiss a degree of complicity in torture. His full ruling is being kept secret at the request of the CPS, the UK intelligence services and Manchester police.

Davis told MPs: "The authorities know full well that this story is an evidential showcase for the policy of complicity in torture."

Referring to the Mohamed case he added: "We are awaiting a police investigation that will presumably end in the prosecution of the frontline officers involved. At the same time, the government are fighting tooth and nail to use state secrecy to cover up crimes and political embarrassments to protect those who are probably the real villains in the piece - those who approved these policies in the first place."