Video report by ITV News Political Correspondent Carl Dinnen
The Government will cooperate with police if they decide to launch a criminal investigation over Gavin Williamson's alleged leak of highly sensitive information regarding Huawei from the National Security Council.
Theresa May's de-facto deputy David Lidington told the House of Commons that the Prime Minister considered the matter closed after Williamson's sacking as Defence Secretary - but Metropolitan Police chief Cressida Dick said the force would look into any evidence it receives regarding the case.
Mr Lidington described the leak as a "fundamental breach of duty" and reassured MPs that Mrs May would not have made the decision to sack Williamson had there not been "compelling evidence".
He said: "The Prime Minister has said she now considers that this matter has been closed and the Cabinet Secretary does not consider it necessary to refer it to the police, but we would of course cooperate fully should the police themselves consider that an investigation were necessary."
Mr Williamson was keeping his own counsel last night - but did post an image of himself with two dogs on Instagram with the caption: "When you have had a pretty tough week it’s rather nice to get out and about with some really good company."
Mr Lidington's remarks were echoed by Cabinet Secretary Sir Mark Sedwill - the most senior civil servant in the UK - who said he did not believe the matter needed to be referred to police.
Metropolitan Police Commissioner Cressida Dick said the Cabinet Office would need to make a referral before police would launch an investigation into allegations Gavin Williamson breached the Official Secrets Act.
Ms Dick said: "We are not going to comment on that particular case suffice to say if the Cabinet Office were to send us a referral at any point that relates to apparent official secrets or leaks we would assess that."
Asked if police needed permission from the Government to launch an inquiry, Cressida Dick said any official secrets would be owned by Number 10 and they would hold any evidence relating to them.
"It is sitting in the Government. We as the police when considering whether there is an appropriate criminal investigation or not will have to be party to that material.
"At the present time we are not in possession."
Deputy Labour leader Tom Watson, who called an urgent question on Thursday, said the severity of the leak required police attention as it may have broken the Official Secrets Act, which in turn would make it a criminal matter.
Mr Watson said: "In response to receiving the most brutal sacking I can think of, (Mr Williamson) has protested his innocence. Therefore this matter cannot be, as the Prime Minister says, closed.
"The essential point here is the Prime Minister has sacked the secretary of state for defence because she believes there is compelling evidence that he has committed a crime. But despite that, she does not believe he should face a criminal investigation - where is the justice in that?"
Liberal Democrat deputy leader Jo Swinson wrote to Ms Dick calling for a criminal investigation.
The letter states: “I am writing to ask you to open a criminal investigation to ascertain whether the actions of Mr Williamson constitute a breach of the Official Secrets Act.”
A Scotland Yard statement given to ITV News suggested police would need to be handed details that criminal activity may have taken place.
Asked if it was on the face of it a breach of the Official Secrets Act, Ms Dick said police would seek advice at an early stage from the Crown Prosecution Service to establish that.
A Downing Street source said it was "not practice in this country for Government to make decisions in relation to prosecutions".
They added: "This was not about what was leaked, it was about where this was leaked from and the importance of maintaining trust and the integrity of the National Security Council."
Williamson strongly denied he is behind the leak about Huawei's potential involvement of the UK's 5G network following his sacking as Defence Secretary on Thursday, saying he would "swear on my children's lives" he was not to blame.
Video report by ITV News Political Correspondent Libby Wiener
On Thursday, Mr Williamson said he would have been “absolutely exonerated” by a police investigation if one were to be launched, after several MPs called for Scotland Yard to launch a criminal probe.
Mr Williamson’s abrupt firing as head of the Ministry of Defence sent shockwaves around Westminster on Wednesday after Theresa May said the inquiry had found “compelling evidence” suggesting he was to blame.
The PM also said she was “concerned” at the manner in which he had engaged with the inquiry by Cabinet Secretary Sir Mark Sedwill.
In a letter, Mr Williamson also revealed that he rejected an offer from Mrs May to resign rather than be sacked, saying that this would have sent a signal that he accepted either he or his team was guilty.
Speaking to the Daily Telegraph, the former minister said he had been “completely screwed”.
“She has got the wrong person and the person who did leak this is still out there,” he said.
The PM’s dramatic firing of her one-time close ally came on the eve of local elections across England and Northern Ireland on Thursday.
The Prime Minister fired Mr Williamson in a 30-minute showdown in her Commons office which triggered a Cabinet reshuffle making Penny Mordaunt Defence Secretary, the first female to occupy the role, and prisons minister Rory Stewart replacing Ms Mordaunt as International Development Secretary.
South Staffordshire MP Mr Williamson, 42, was a surprise appointment as defence secretary in November 2017 after a meteoric rise which saw him enter the Cabinet without ever having served in a junior ministerial role.
He was one of Mrs May’s closest allies after she made him chief whip on entering Downing Street in 2016.
But during his time in the Cabinet, he showed increasing signs of independence from the PM and was widely regarded as preparing for a tilt at the top job when she stands down.