GCHQ and the NSA have reportedly been spying on e-mail exchanges between MPs and their constituents as a matter of course for the last few years.

Documents released by spook whistleblower Edward Snowden in 2013 revealed details of the top secret Tempora scheme, which allowed the British intelligence agency to intercept data travelling on backbone Internet cables crossing the Irish Sea and English Channel. Bulk storage of this data by the UK's eavesdropping nerve centre GCHQ is allowed under current law.

According to a Computer Weekly report, co-written by acclaimed investigative journalist Duncan Campbell, parliament's switch to Microsoft e-mail cloud services (Office 365) in 2014 means that even UK-to-UK communications often travel via Redmond’s data centres in Ireland and the Netherlands. That's the conclusion of a study carried out by the IT publication. It tracked the path of hundreds of MPs’ e-mails, and found that 65 percent of those messages were routed overseas.

“Every message also contained references to having been passed through clusters of scanning computers connected to GCHQ and located in the UK, France, and Germany,” Computer Weekly said.

While the Tempora system “only” collects metadata, including sender, recipient, and subject line, the e-mails can also be scanned for “keywords” through a network run by Symantec-owned MessageLabs, which provides spam filtering and malware detection services to parliament.

According to Snowden’s disclosures, a secret cyber security project dubbed "Haruspex" allows GCHQ operatives to use MessageLabs’ abilities to scan e-mails for “national security” purposes.

Meanwhile the US National Security Agency’s (NSA) notorious Prism system grants it access to parliamentary e-mail and documents directly through orders given to Microsoft.

Both Microsoft and MessageLabs declined to comment on this story when approached by Ars on Thursday morning. At time of publication, no response had been received from GCHQ's press team.

MPs will discuss the controversial Investigatory Powers Bill when it returns to the floor of the House of Commons on Monday for its report stage and third reading. The proposed law would give sweeping new powers to the UK's security services.