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: “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.”
Two Cabinet ministers, Defence Secretary Gavin Williamson and Foreign Secretary Jeremy Hunt, publicly denied they were responsible.
Sources close to International Development Secretary Penny Mordaunt and International Trade Secretary Liam Fox denied their involvement.
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 Metropolitan Police spokeswoman said she could neither confirm nor deny whether any police investigation had been requested or was under way, and Downing Street sources refused to comment on security matters.
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 a national newspaper 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.
Five ministers – Mr Hunt, Mr Williamson, Home Secretary Sajid Javid, Ms Mordaunt and Dr Fox – were reported as having expressed concern with the move.
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.