A former head of the civil service has condemned the leak of secret discussions by the UK’s top national security body as a “complete outrage” as his successor demanded ministers at the meeting confess or deny any involvement.

Lord O’Donnell said the disclosure of discussions on whether to grant Chinese telecoms giant Huawei the go-ahead to participate in the UK’s 5G communications network was “incredibly serious” and applauded Sir Mark Sedwill’s actions.

Sir Mark, who is also the national security adviser, is understood to have sent a letter ordering those present to tell him “immediately” whether they were involved.

Prime Minister Theresa May is facing calls for a full police inquiry to find the perpetrator, while MPs have denounced the unprecedented leak following Tuesday’s meeting of the National Security Council (NSC) as “completely shocking”.

Lord O’Donnell told BBC Radio 4’s Today programme: “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’ – this is really important for the country, these issues are massively important.”

Foreign Secretary Jeremy Hunt (left) and Sir Mark Sedwill
Foreign Secretary Jeremy Hunt (left) and Sir Mark Sedwill arrive in Downing Street for a Cabinet meeting (Stefan Rousseau/PA)

Two Cabinet ministers, Defence Secretary Gavin Williamson and Foreign Secretary Jeremy Hunt, publicly denied that they were responsible.

Sources close to International Development Secretary Penny Mordaunt and International Trade Secretary Liam Fox denied their involvement.

Chancellor Philip Hammond said it was important to get to the bottom of what happened, telling the BBC: “To my knowledge there has never been a leak from a National Security Council meeting before and therefore I think it is very important that we get to the bottom of what happened here.

“It’s not about the substance of what was apparently leaked – it’s not earth-shattering information – but it is important that we protect the principle that nothing that goes on in National Security Council meetings must ever be repeated outside the room.”

However, former defence secretary Sir Michael Fallon said a Whitehall leak inquiry by civil servants was insufficient and that only a 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.

A Met Police spokeswoman said she could neither confirm nor deny any police investigation had been requested or was under way, and Downing Street sources refused to comment on security matters point blank.

Former minister Andrew Mitchell said Mrs May should order MI5 to conduct a full investigation, which could include interviewing Cabinet ministers if necessary.

The anger among MPs reflected concerns that the leak from the NSC – where senior ministers are briefed by intelligence chiefs from MI5, MI6 and GCHQ – could damage intelligence-sharing relations with key allies.

It followed a report in The Daily Telegraph that Mrs May, who chaired the meeting, had overridden the objections of key ministers to give the green light for Chinese tech giant Huawei to participate in the UK’s 5G communications network.

Sir Michael Fallon
Sir Michael Fallon has called for a full police inquiry into the NSC leak (Stefan Rousseau/PA)

Five ministers, Mr Hunt, Mr Williamson, Home Secretary Sajid Javid, Ms Mordaunt and Dr Fox, were reported as having expressed concern.

David Lidington, the Cabinet Office Minister and Mrs May’s de facto deputy, Chancellor Philip Hammond, Business Secretary Greg Clark and Attorney General Geoffrey Cox also sit on the NSC, while it is thought other ministers attend when it is relevant to do so.

MPs immediately linked the disclosure to the manoeuvrings around the Tory leadership, with whoever was responsible seeking to burnish their credentials as being “tough on China”.

At a lunch for Westminster journalists, Mr Hunt flatly denied that it was him.

“I think it is utterly appalling that that should happen,” he said.

Jeremy Hunt
Jeremy Hunt has denied he was responsible for the NSC leak (Victoria Jones/PA)

“I have never leaked confidential Cabinet discussions and I never will.”

In a statement, Mr Williamson said neither he nor any of his team had “divulged information from the National Security Council”.

Mr Javid, visiting a police training college in Kent, said it was “completely unacceptable” for any minister to release sensitive information and that it should “absolutely be looked at”.

Calls for an inquiry were backed by former national security adviser Lord Ricketts, who said it was the first major leak from the NSC since its inception in 2010.

He suggested investigators from the Security Service, MI5, could be brought in “to make the culprit feel very uncomfortable”.