Cabinet ministers could face having their mobile phone records trawled through in a leak inquiry, an ex-civil service chief has suggested as he branded the unprecedented breach a "complete outrage".
Sir Gus O'Donnell's remarks came as his successor as cabinet secretary reportedly launched a formal inquiry into the leak of the highly-secretive National Security Council (NSC) and issued an ultimatum to ministers.
Alongside intelligence chiefs, ministers had discussed whether to grant the Chinese telecommunications giant Huawei permission to work on the UK's 5G mobile network earlier this week.
Lord O'Donnell, the head of the civil service between 2005 and 2011, who helped set up the NSC under David Cameron, described the leak of highly-classified material from the meeting this week as "incredibly serious".
"If I were cabinet secretary - and I thoroughly applaud what my successor Mark Sedwill has done - is to say look this is just beyond the pale," he told the BBC's Radio 4 Today programme. "This is really important for the country. These issues are massively important."
Asked what the cabinet secretary could do in response to the leak, he said: "You can get someone who has the security clearance to do this work. That's the first thing and then look people in the eye and find out from them are they telling the truth.
"There are various things you can do - you have to do everything within the law. I've been inquiries where we've looked at mobile phone records, email records and the like."
He continued: "Personally I would be doing that. You can imagine secretaries of states saying it wasn't me - I didn't talk to any journalists whatsoever. Of course it might well be that they spoke to someone and that someone spoke."
The comments from the well-respected former civil servant also followed for Theresa May to launch a full police into Huawei leak and the culture secretary Jeremy Wright said the government could not exclude the possibility of a criminal investigation.
Downing Street refused to say on Thursday whether a leak inquiry was already under way but insisted the prime minister regarded the protection of information concerning national security as a "matter of the highest importance".
Two cabinet ministers - defence secretary Gavin Williamson and foreign secretary Jeremy Hunt - publicly denied that they were responsible.
However, former defence secretary Sir Michael Fallon said a Whitehall leak inquiry by civil servants was insufficient and that only a proper Scotland Yard investigation could get to the truth.
He said cabinet ministers attending the meeting should have their mobile phones checked to see if they contacted journalists afterwards, and if anyone was found to be responsible they should be prosecuted under the Official Secrets Act.
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