MI6 has been forced to reveal documents detailing how it may access legally privileged communications between solicitors and their clients, even if the lawyers are suing the government.

Policy guidance handed over to the civil liberties organisation Reprieve shows how the Secret Intelligence Service (SIS) is attempting to regulate its mass surveillance practices and demonstrate compliance with the law.

The revelations have emerged from a case brought by lawyers for two Libyans, Abdel-Hakim Belhaj and Sami al-Saadi, who, along with their families, were abducted in a joint MI6-CIA operation and sent back to Tripoli to be tortured by Colonel Muammar Gaddafi’s regime in 2004.

Their complaint about illegal monitoring is being heard before the investigatory powers tribunal and a full trial of the issues is expected this spring.

Exchanges between lawyers and their clients enjoy a special protected status under UK law. Following exposure of widespread monitoring by the US whistleblower Edward Snowden in 2013, Belhaj’s lawyers feared that their exchanges with their clients could have been compromised by GCHQ’s interception of phone conversations and emails.

To demonstrate that its policies satisfy legal safeguards, MI6 has been required to disclose internal guidance on how intelligence staff should deal with material protected by legal professional privilege.

The papers note: “Undertaking interception in such circumstances would be extremely rare and would require strong justification and robust safeguards. It is essential that such intercepted material is not acquired or used for the purpose of conferring an unfair or improper advantage on SIS or HMG [Her Majesty’s government] in any such litigation, legal proceedings or criminal investigation.”

The internal documents also refer to a visit by the interception commissioner (Sir Anthony May) last summer to examine interception warrants, where it was discovered that regulations were not being observed. “In relation to one of the warrants,” the document explained, “the commissioner identified a number of concerns with regard to the handling of [legal professional privilege] material”.

The documents also contain references to “bin lists” – apparently material, such as lawyers’ phone numbers, that can be discarded as irrelevant. Reprieve is considering applying to have its telephone numbers put on this list to avoid being targeted.

Commenting on the latest document releases, Cori Crider, a lawyer who represents Belhaj, said: “MI6’s brand-new eavesdropping policy still has serious problems – it still envisages that MI6 will snoop on private legal calls even in cases where it is being sued for torture.

“But these issues only highlight the double-decker-sized loopholes that were in place when Mr Belhaj and his wife were preparing their legal claims. This last-minute effort by MI6 to clean up their act shows Reprieve was right to fear that our private communications with torture victims, and possibly with the police in Operation Lydd [the investigation into the Libyan renditions], were compromised.

“There can be no justification for spying on our privileged calls. If spying took place, and information leaked, the government must come clean about it immediately so we can begin to set this family’s torture trial back on a fair footing.”