Two years ago Edward Snowden let citizens know that their privacy wasn’t all it seemed. Records were routinely being kept on the websites they visited, the texts they sent and the numbers they called. Even search terms and passwords could sometimes be harvested as “bulk data”, making it possible in principle to weave an intimate portrait from disparate electronic traces.

There were shockwaves around the world, from Washington to Berlin. Westminster, however, shrugged off the news, with many MPs more interested in taking pot-shots at Mr Snowden, and sometimes the Guardian, than in engaging with the substance of what he had to say. If parliamentarians were less than excited about snooping, then – on the-personal-is-the-political principle – it could be because they didn’t imagine that it affected them. The Wilson doctrine – the 50-year-old prime ministerial promises that MPs’ communications wouldn’t be tapped – gave that hunch some basis. Today, however, the investigatory powers tribunal (IPT) told them bluntly that the doctrine had no force in law. Now it is the politicians’ turn to discover that their privacy isn’t all that it had seemed.

A court that has often been criticised for its secrecy, the IPT arrived at its bald conclusion by way of an argument that in places seemed almost contemptuous towards elected politicians. The judges approvingly summarised the view of the intelligence agencies’ lawyer as being that Harold Wilson’s original vow not to tap MPs’ phones was merely “a political statement in a political context, encompassing the ambiguity that is sometimes to be found in political statements”. They dusted down and cited an old review by Sir Swinton Thomas which had argued for the abolition of the doctrine, almost as if this had some special authority, whereas in fact it was expressly rejected by the prime minister of the day, Tony Blair, who indeed extended the old bar on tapping members’ phones to cover email.

The IPT dismissed out of hand any special exemption from bulk data collection, and when it comes to the state targeting the substance of MPs’ communication – that is, reading emails or listening to calls – it was content to rely on the safeguards provided by internal codes of practice in the intelligence agencies. Remarkably, however, these codes appear to have been written down and revised only because of this case. Their protections are heavily caveated – sometimes implying that it is only MPs’ constituency work that is privileged, and always emphasising the lack of any absolute bar. Internal guidelines are always subject to convenient tweaks; one that emerged during the case, for example, dashed hopes at Holyrood that MSPs might be protected by clarifying that there were no Wilson protections at all beyond Westminster.

Some might say MPs deserve no more privacy than anyone else; others will argue, with good reason, that since the executive will have special, political reasons to pry on legislators, special checks are required. We can have the debate, but surely we shouldn’t dismiss a principle of 50 years’ standing without it. Self-respecting MPs cannot casually let go of a promise that successive prime ministers have given to them. If they want protections, they must now write them into the law – and, while they’re at it, they might start giving a bit more thought to everybody else’s privacy as well.