SNP leader Nicola Sturgeon is seeking "urgent" assurance from David Cameron that members of Scotland's parliament are not being spied on following new snooping revelations on Friday (24 July).

Early on Friday, Scotland's Daily Record newspaper revealed documents showing Britain's intelligence services were cleared in March to target Holyrood MSPs.

"Will you give an immediate assurance that this policy change by GCHQ will be reversed and that the Wilson doctrine will again be applied to MSP communications?" Sturgeon asked in a letter to Cameron sent Friday. As Scotland's First Minister she heads Scottish government.

Established in 1966, the Wilson Doctrine offers legal protection to the communications of MPs, MSPs, and peers, from interception by police and security services. Former PM Tony Blair clarified in 1997 that it also covers digital communications — such as emails.

The Scottish government was not consulted on changes to the rules, Sturgeon reveals.

"I am sure you will agree with me that, excepting truly exceptional circumstances involving national security, the confidentiality of communications between parliamentarians and their constituents is of the utmost importance," Sturgeon told Cameron.

"Are these reports correct in stating that there has been a change of policy and that GCHQ has ceased to apply the Wilson Doctrine to the communications of MSPs?" she asked the PM.

"We need clarification. That's a responsible first step," said a source within the SNP. "We need to know how much of this story is true. It looks absolutely plausible and sourced."

The Investigatory Powers Tribunal is hearing a case brought by parliamentarians on the issue of the Wilson Doctrine today.

Ahead of the hearing, Green Party MP Caroline Lucas said "the blanket surveillance of the communications of Parliamentarians could have a deeply chilling effect on our relationship with the public."

She called MPs and peers "a trusted source for whistle-blowers and those wishing to challenge the actions of the government."

Early this year the SNP's chief of staff, who has since left the position, said online that he believed there is a "good chance" his emails to journalists were hacked by GCHQ. The revelations mean the MSPs communications could have been collected during the general election 7 May.

Good chance my emails to 'defence journalists' hacked by GCHQ. Hacked for holding Government to account. http://t.co/Zj1Lq8daoO — Luke Skipper (@LJ_Skipper) January 20, 2015

Several cases have emerged in the past year showing that MPs phone calls and other communications have been targeted by UK security services.

In November 2014, then interim Interception of Communications Commissioner, Sir Paul Kennedy, told a parliamentary committee that the Wilson Doctrine "ought to be abrogated" and that the law doesn't apply anymore.

For now, Sturgeon wants to know "why was this decision taken, when and by whom was it taken, and was there any ministerial knowledge or approval?"

The Prime Minister's office was not available to immediately respond. The Cabinet Office said "we are not going to comment during ongoing litigation".