The tribunal said it was satisfied that assurances given around the surveillance of MPs by prime minister Harold Wilson in the 1960s, which have been reasserted by subsequent governments, are “not enforceable in English law”.

In its judgment, the tribunal acknowledged that the policy had been repeated and clarified by Tony Blair and Gordon Brown during their respective premierships, and by Home Secretary Theresa May as recently as July 2015.

However, it ruled the doctrine “does not operate so as to create a substantive legitimate expectation” and has “no legal effect”.

The decision was described as a “body-blow for parliamentary democracy” by Caroline Lucas MP, one of the claimants in the case.

She added: “My constituents have a right to know that their communications with me aren’t subject to blanket surveillance – yet this ruling suggests that they have no such protection.

“Parliamentarians must be a trusted source for whistleblowers and those wishing to challenge that actions of the government. That’s why upcoming legislation on surveillance must include a provision to protect the communications of MPs, peers, MSPs, AMs and MEPs from extra-judicial spying.”

Lady Jones, another claimant, said: “As parliamentarians who often speak to whistleblowers – from campaigners whose groups have been infiltrated by the police to those exposing corruption in government departments – this judgment is deeply worrying.

“Our job is to hold the executive to account, and to do that effectively it’s crucial that people feel they can contact us without their communications being monitored.

“In a democracy there is absolute no excuse for people who contact parliamentarians to be subject to blanket surveillance by the security services.”