CALLS for a full criminal investigation have been made after Theresa May sacked Gavin Williamson, the Defence Secretary, as the person behind the leak of information from the National Security Council.

He learnt his fate in a half-hour face-to-face meeting in the Prime Minister’s House of Commons office following PMQs, when she told him that a leak inquiry had found "compelling evidence" of his involvement.

READ MORE: Gavin Williamson sacked from government over Huawei leak 

Mr Williamson denied either he or his staff were behind the unprecedented breach of security, which revealed Mrs May’s controversial decision to give Chinese tech giant Huawei a role in building Britain’s 5G digital network.

Last night, he continued to plead his innocence, suggesting that the Whitehall probe had been rushed.

Labour, the SNP and Liberal Democrats all called for a Scotland Yard probe to be launched into whether the former Cabinet minister had breached the Official Secrets Act.

However, Downing Street made clear the Prime Minister considered the matter "closed".

The PM swiftly appointed Penny Mordaunt, the International Development Secretary, as Mr Williamson’s replacement at the Ministry of Defence, becoming Britain’s first female Defence Secretary. Her role is taken over by Rory Stewart, the former Black Watch officer, who is promoted to the Cabinet from his job as Prisons Minister.

Last week’s leak of information from the NSC to the Daily Telegraph caused widespread anger and dismay across Whitehall as it undermined the confidentiality that ministers and senior security officials rely upon to discuss highly classified material.

One insider suggested it was leaked to make the person who did so appear “tough on China” as ministers manoeuvred to succeed the PM in the forthcoming Tory leadership contest.

The official investigation was launched by Sir Mark Sedwill, the Cabinet Secretary and National Security Advisor, who was said to have been determined to get to the bottom of the deeply embarrassing security breach.

It was suggested the phone calls and emails of ministers and their special advisors were examined to find the culprit.

READ MORE: Gavin Williamson sacking just further proof of crisis-ridden Government 

In an unusually frank letter, Mrs May said she was saddened and disappointed by the turn of events, which meant she could “no longer have full confidence” in Mr Williamson as a Cabinet member.

The PM told him she was “concerned by the manner in which you have engaged with this investigation”.

Mrs May explained how other NSC attendees had "all answered questions, engaged properly, provided as much information as possible to assist with the investigation, and encouraged their staff to do the same".

But the PM claimed: "Your conduct has not been of the same standard as others.”

She stressed Sir Mark’s inquiry had provided “compelling evidence, suggesting your responsibility for the unauthorised disclosure. No other, credible version of events to explain this leak has been identified".

Downing St later declined to go into detail about what the compelling evidence was but stressed the PM “considers the matter closed”.

But in his letter of response, Mr Williamson told Mrs May: "I am sorry you feel recent leaks from the NSC originated in my department. I emphatically believe this was not the case."

He went on: "I strenuously deny that I was in any way involved in this leak and I am confident that a thorough and formal inquiry would have vindicated my position."

The 42-year-old revealed he had turned down an offer from the PM for him to resign rather than be sacked but he pointed out this would have implied he had accepted either he or his team were responsible for the leak.

Scotland Yard said in a statement: "We're aware of the media reports in relation to the leak and that is a matter for the National Security Council and the Cabinet Office to look at.

"At this time, we're not carrying out an investigation. Clearly, if at any stage we receive any information that would suggest criminal offences have been committed, then we will look into that."

However, opposition parties demanded a police probe.

Tom Watson, Labour’s deputy leader, said: "If he has leaked from the National Security Council, Gavin Williamson should be prosecuted under the Official Secrets Act. And he should forgo his ministerial severance pay."

Stewart McDonald for the SNP said: “It is completely unacceptable the Tories are putting the interests and national security of Scotland and the UK at risk in an increasingly dirty and shameful leadership battle.

"The UK Government must clarify if they believe Mr Williamson has broken the law, and if so, the matter must be passed to the police for a full criminal investigation.”

Sir Vince Cable, the Liberal Democrat leader, said the leak saga could not begin and end with dismissal from office.

“What is at stake is the capacity of our security services to give advice at the highest level. This must now be referred to the Metropolitan Police for a thorough criminal investigation into breaches of the Official Secrets Act,” he insisted.

Jeremy Hunt, the Foreign Secretary, commenting on Mr Williamson's departure as he arrived in Addis Ababa in Ethiopia, said: "On a personal level, I'm very sorry about what happened for Gavin's sake but given the gravity of the situation there was no other alternative outcome."

Mr Williamson 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.

However, 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 eventually stands down.