A decision that the convention preventing the tapping of MPs' telephones has no legal basis has been branded a "body blow for parliamentary democracy".

Green Party MP Caroline Lucas who, with Baroness Jones of Moulsecoomb and former MP George Galloway, took a complaint to the Investigatory Powers Tribunal, attacked its findings on the "status, meaning and effect" of the Wilson doctrine.

Implemented by prime minister Harold Wilson in November 1966, it lay down the policy of no tapping of the phones of MPs or members of the House of Lords, unless there is a major national emergency, and that any changes to the policy will be reported by the prime minister to Parliament.

Lawyers allege that the politicians' communications are being intercepted by GCHQ as part of the Tempora programme, which monitors and collates on a blanket basis the full range of electronic communications data produced in, or transiting through, the UK and other countries.

Today, the IPT made declarations that the doctrine applies only to targeted, and not incidental, interception of Parliamentary communications, but that it has no legal effect, save that in practice the Security and Intelligence Agencies must comply with their own guidance.

Its President, Mr Justice Burton said the IPT was satisfied that the doctrine was not enforceable in English law by the claimants or other MPs or peers by way of legitimate expectation.

Ms Lucas said: "This judgment is a body blow for parliamentary democracy. 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 whistle blowers and those wishing to challenge the 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."

Baroness Jones of Moulsecoomb said: "As parliamentarians who often speak to whistle-blowers - 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 absolutely no excuse for people who contact parliamentarians to be subject to blanket surveillance by the security services."

Rosa Curling from lawyers Leigh Day said: "Promises made by successive prime ministers about the Wilson doctrine were not worth the paper they were written on. Members of the Houses of Parliament have, for the past half century, relied upon promises made by prime ministers that their communications would be properly protected. Today's judgment shows that they were wrong to place any reliance on these assurances."

A Government spokesman said: "The Government welcomes the ruling given today by the Investigatory Powers Tribunal (IPT). The IPT has comprehensively rejected the claim brought by a number of Parliamentarians that their communications were improperly intercepted and has found that all activity has been within the law."

The Prime Minister's official spokeswoman said the doctrine remained in place but it had been made clear "that it is possible for the intelligence agencies to act in a way which is targeted" in some circumstances.

The doctrine "exists alongside and should be considered as part of their approach", she told reporters, rejecting the suggestion that it had become meaningless.

"The point here is that it does exist, the Wilson doctrine, and it has been recognised that it should - but that should not undermine the ability of the intelligence agencies where they need to act in a targeted way.

"It may be that the intelligence agencies are targeting communications from a specific individual who is then in touch with an MP.

"The intelligence agencies are focusing on the individual, it does not mean they are targeting the MP."

Asked if any such interception of MPs' communications required the specific approval of the Home Secretary, she said: "There is clearly a system of ministerial warrants in place."

Liberal Democrat home affairs spokesman Alistair Carmichael said: "This ruling could have a chilling effect on democracy in the UK. MPs, MSPs and other elected representatives must have the ability to talk to their constituents without fear that their private communications are being routinely monitored by intelligence agencies."

He called for "urgent answers" from the Government and added: " The Liberal Democrats will be demanding journalists' sources, legal professional privilege and protection of the communications of democratically-elected representatives are all given a strong legal basis and protection in the new Investigatory Powers Bill."