As part of an ongoing legal action, the MI5, MI6 and GCHQ intelligence services have disclosed their guidelines for snooping on lawyers.
All three allow lawyers' communications to be intercepted in some circumstances. Rules for GCHQ staff say "you may, in principle, target the communications of lawyers," but adds "you must give careful consideration to necessity and proportionality."
The information was disclosed in court documents after a request from lawyers for two Libyan men who accuse British spies of complicity in their detention and rendition to their homeland a decade ago.
The men say British spies infringed their right to a fair trial by intercepting privileged communications.
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The intelligence agencies say their guidelines prevent their lawyers from seeing material about cases in which they are involved.
But legal charity Reprieve, which represents the Libyans, said those procedures were only put in place recently and are too lax.
"It's now clear the intelligence agencies have been eavesdropping on lawyer-client conversations for years," said Cori Crider, a director of Reprieve and a lawyer for the Libyans and their families.
Al-Saadi and his family accepted USD 3.5 million from the British government in 2012 to settle a claim that the UK approved his 2004 rendition to Moammar Gadhafi's Libya, where he was imprisoned and tortured. The lawsuit by Belhaj is continuing.