British spy agencies have secret policies on the interception of private communications between lawyers and their clients, attorneys told a judge Thursday, arguing that the documents should be made public.

The lawyers — acting on behalf of two men who claim they were kidnapped under a UK rendition program, taken to Libya, held captive, and tortured — demanded the documents in a rare public hearing of the UK's secretive Investigatory Powers Tribunal (IPT).

The two men, Abdel Hakim Belhaj and Sami al-Saadi, are represented by lawyers from Reprieve, a human rights NGO.

Belhaj is a Libyan dissident who led a failed insurgency against former dictator Muammar Qaddafi before fleeing to Malaysia. He was kidnapped in 2004 — allegedly by CIA agents working in cooperation with British and Libyan officials — along with his pregnant wife and flown via Thailand to Tripoli, where he was held until his release in 2010. Documents discovered during the 2011 Libyan revolution corroborated his story.

Al-Saadi was another opponent of Qaddafi who fled to Hong Kong. He was also picked up in 2004 along with his wife and four young children in an alleged joint UK-US-Libyan operation and forcibly returned to Libya, where he was imprisoned and tortured. Al-Saadi previously received a payout of more than $3.5 million from the UK government to settle a lawsuit over the rendition.

Lawyers for the British intelligence agencies said in January that they would not read intercepted communications between the rendition victims and their legal team, but they also refused to say whether previous correspondence had been examined.

Belhaj and al-Saadi maintain that the UK government spied on their client-lawyer communications, damaging their right to a fair trial in previous kidnap and torture compensation claims.

Client-lawyer communications are "privileged" under longstanding rules in British courts. But in documents discussed at the hearing Thursday, the government admitted that its intelligence agencies have policies that provide "guidance" on the interception of privileged material by security service personnel.

The UK government maintains that releasing the "guidance" documents would be "damaging to the public interest or prejudicial to national security, the prevention or detection of serious crime and the continued discharge of the functions of the intelligence services."

A document summarizing the security policies said that each intelligence agency — MI5, MI6 and GCHQ — had developed its own procedures.

Further preliminary hearings will be held in the case to decide whether the documents should be disclosed, and whether the tribunal even has the power to order the government to release the documents.

Although the hearing Thursday was open, Dinah Rose, an attorney representing the Libyans, raised concerns about its accessibility to the public. Rose questioned the fact that her clients were initially asked to provide a list of attendees, and that the proceeding was scheduled in the evening outside of normal hours.

"This tribunal, of all tribunals, should bend over backwards to ensure that it is sitting properly in public," Rose said.

IPT hearings, which investigate complaints about the country's intelligence agencies, are usually held in secret, but the "notice of complaint," brought by the law firm Leigh Day on behalf of Reprieve and the Libyans, calls for all hearings to be held in open court.

On Tuesday, former home secretary David Blunkett called for judicial oversight of the UK's intelligence agencies to be made much more robust and transparent. Blunkett, who helped introduce new regulations on UK surveillance in 2000, also called for the legal framework on mass surveillance to be updated on a regular basis.

"Sometimes we have to be more sophisticated than saying 'We can't tell you anything because it might be a danger,'" Blunkett said. "That actually undermines confidence and consent. It is real old-fashioned paternalism because it is 'We know but you mustn't know.'"

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