Documents showed that safeguards to prevent spy station GCHQ from tapping the phones of MPs or hacking their e-mails no longer included their colleagues in the Scottish Parliament.
First Minister Nicola Sturgeon demanded an “immediate assurance” that the policy change will be reversed and said MSPs should be treated in the same way as Westminster politicians.
In a letter to the Prime Minister, Ms Sturgeon said: “Excepting truly exceptional circumstances involving national security, the confidentiality of communications between parliamentarians and their constituents is of the utmost importance. I am sure you will also agree that it is just as important for MSPs as it is for MPs.”
She also called on Mr Cameron to say whether any MSPs’ communications had been intercepted and if ministers knew about the decision to change the guidelines.
Acting Scottish Labour leader Iain Gray said the decision to lift the ban on eavesdropping on MSPs was “outrageous”.
He said: “It is utterly unacceptable for the communications between devolved representatives across the UK and their constituents to be monitored by GCHQ.
“There needs to be full transparency from the UK government on this. We need to know urgently who decided on this major rule change and when. For the rules on spying on elected representatives across the UK to change without any sort of public scrutiny or accountability is a democratic outrage.”
Edinburgh South Labour MP Ian Murray has tabled a series of parliamentary questions in the House of Commons in a bit to get to the bottom of the change.
Holyrood Presiding Officer Tricia Marwick has also written to the Prime Minister seeking clarification.
Her letter said: “I feel strongly that all elected members should be treated in the same way, regardless of which parliament or assembly they are elected to, especially with regard to any communications a member has with his or her constituents.”
Amnesty International warned people may be put off contacting their MSP if they think their communications could be spied on.
Scottish Green leader Patrick Harvie said the change of guidelines to allow spying on MSPs was “deeply disturbing”.
He said: “The privacy of parliamentary communications is a vital tool to protect the public interest.”
Scottish Liberal Democrat justice spokeswoman Alison McInnes said: “The security services should be focused on catching terrorists and not spying on MSPs.
“We need to know why this step has been taken and if it was approved by Tories at the top of government.”