The revelation – one of the biggest privacy shocks to emerge post-Snowden – came when GCHQ, MI5 and MI6 were forced to hand over their secret guidelines for monitoring lawyers as part of an Investigatory Powers Tribunal case brought against the Government by UK legal charity Reprieve.
It was “suspicious technical problems” that first alerted the defence lawyers to possible snooping on their private discussions with their clients - the families of two Libyan men who have accused the UK of helping then-president Gaddafi to abduct and ‘render' opponents back to Libya.
Possibly clarifying this, Cori Crider, a Reprieve director and US counsel for the Belhadj and al Saadi families, told the New York Times: “Often when I call my clients in Libya, the original ring is an American one.”
Their suspicions were confirmed by the secret documents, released on Thursday, which show the UK intelligence agencies ignoring the principle of ‘legal professional privilege' (LPP), which dates back to at least the 16th century.
In its guidance, GCHQ tells agents “you may, in principle, target the communications of lawyers” though it advises them to “give careful consideration to necessity and proportionality”.
MI5 tells its officers that “in principle, and subject to the normal requirements of necessity and proportionality, LPP material may be used just like any other item of intelligence”.
Reprieve, meanwhile, says MI6 offers almost no guidance for its agents on the interception of legally privileged material.
And the three agencies admit their phone and email intercepts may have been wrongly used to influence at least one security-related court case.
The documents say: “There is a single known case/instance, from the material which has been examined, where the potential for ‘tainting' was identified. Additional procedures were promptly put in place to safeguard the integrity of the litigation.”
And according to The Daily Telegraph, ‘redacted' parts of the documents suggest the three agencies may also have intercepted sensitive communications with other professions, such as judges and politicians.
Senior Conservative MP David Davis has called the revelations “a national scandal”.
Cori Crider at Reprieve told journalists: “It's now clear the intelligence agencies have been eavesdropping on lawyer-client conversations for years. Today's question is not whether, but how much, they have rigged the game in their favour in the ongoing court case.
“The documents clearly show that MI5's and GCHQ's policies on snooping on lawyers have major loopholes. And MI6's ‘policies' are so hopeless they appear to have been jotted down on the back of a beer mat. This raises troubling implications for the whole British justice system. In how many cases has the Government eavesdropped to give itself an unfair advantage in court?”
Reaction within the UK security and legal communities has been mixed.
Alan Woodward, a Europol adviser and visiting professor at Surrey University's Department of Computing, told SCMagazineUK.com: “On the one hand one can understand it's a valuable source of intelligence in trying to head off potential attacks. The very people that the intelligence agencies know may be involved in commissioning terrorist attacks may be talking to their lawyers about it.
“But the danger is that privileged information will leak back into the justice system and be used against someone in the courts, and there is some evidence of that in one case.
“There's got to be cast-iron ways of making sure it doesn't leak back. There should be no way for the information to go back to the legal team that's prosecuting that person. That's just so against our principles of how our legal system works.”
Woodward added: “They've got to redouble their efforts if they're going to continue to do it. It's in everyone's interest to make sure the right processes are in place, because you don't want a case failing because the prosecution have had access to privileged conversations.”
But he said lawyers too have a duty to do more. He told SC: “Maybe the legal profession needs to start making more use of things like encryption. They have a duty to make sure their conversations remain privileged so they've got a duty here as well.”
For its part, the Law Society has said existing legislation protection for lawyers is “inadequate”.
A spokesperson told journalists: “The absence of explicit protection for legal professional privilege in the Regulation of Investigatory Powers Act 2000 (RIPA) has been of long-standing concern to the Law Society and we have raised our concerns with the Home Office.
“The documents released in this case before the tribunal illustrate the inadequacy of the existing legislative framework. We hope they will be fully taken into account by the Independent Reviewer of Terrorism Legislation in considering the case for amending or replacing RIPA.”
The revelations come just a few weeks after American whistle-blower Edward Snowden urged lawyers and other professions to routinely encrypt their data to protect against mass surveillance.
The news also comes in the same week as new GCHQ head, Robert Hannigan, urged US tech firms like Facebook and Twitter to cooperate more with spy agencies, and accused them of being “in denial” about their services being used by terrorists as their "command and control networks of choice".