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."

Inside Huawei's new campus for workers in China Show all 20 1 /20 Inside Huawei's new campus for workers in China Inside Huawei's new campus for workers in China An employee enters a train in the Huawei's Ox Horn campus at Songshan Lake in Dongguan Reuters Inside Huawei's new campus for workers in China A worker cleans a waterway as office buildings are seen at Huawei's new Ox Horn Research and Development campus in Dongguan, near shenzen Getty Inside Huawei's new campus for workers in China An area of Huawei's Ox Horn campus modelled after Cesky Krumlov in Czech Republic Getty Inside Huawei's new campus for workers in China The real Cesky Krumlov in Czech Republic Getty Inside Huawei's new campus for workers in China Employees sleep in their cubicle in the research and development area after lunch at the Bantian campus in Shenzhen Getty Inside Huawei's new campus for workers in China An area of Huawei's Ox Horn campus modelled after Heidelberg in Germany Getty Images Inside Huawei's new campus for workers in China Employees play basketball on a court within the staff housing complex at the end of the workday at the Bantian campus in Shenzhen Getty Inside Huawei's new campus for workers in China A replica of the Karl Theodor Bridge in Huawei's Ox Horn campus Getty Inside Huawei's new campus for workers in China The real Karl Theodor Bridge in Heidelberg, Germany Getty Inside Huawei's new campus for workers in China Huawei's Ox Horn campus at Songshan Lake in Dongguan Reuters Inside Huawei's new campus for workers in China Employees ride the bus home at the end of the workday from the company's Bantian campus in Shenzhen Getty Inside Huawei's new campus for workers in China Huawei workers eat their subsidised lunch in one of many large cafeterias at the company's Bantian campus in Shenzhen Getty Inside Huawei's new campus for workers in China Reception staff walk in front of a large screen showcasing different technologies in the foyer of a building used for high profile customer visits at the campus in Shenzen Getty Inside Huawei's new campus for workers in China An employee reads in the staff library on a break at the company's Bantian campus in Shenzhen Getty Inside Huawei's new campus for workers in China An area of Huawei's new Ox Horn campus modelled after a European City Getty Inside Huawei's new campus for workers in China Servers are seen inside Huawei's factory campus in Dongguan Reuters Inside Huawei's new campus for workers in China The conductor waits for a train in the Huawei's Ox Horn campus at Songshan Lake in Dongguan Reuters Inside Huawei's new campus for workers in China A general view shows the research and development centre at Huawei's Ox Horn campus AFP/Getty Inside Huawei's new campus for workers in China Employees works on a mobile phone production line at Huawei's factory campus in Dongguan Reuters Inside Huawei's new campus for workers in China An office building on the Huawei campus in Dongguan AFP/Getty

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.