Pressure builds on Theresa May to call in Scotland Yard as 'matter of urgency' over security leak

It is believed that Sir Mark Sedwill, the Cabinet Secretary and National Security Advisor, who oversaw the Whitehall leak inquiry, does not believe the breach of confidence attributed to Mr Williamson is tantamount to a crime.

No 10 stressed: “This investigation was not about what was leaked, it was about where it was leaked from and the importance of maintaining trust and the integrity of the NSC.

“The PM’s letter[to Mr Williamson] makes clear the decision was driven by his conduct surrounding the investigation and the compelling evidence suggesting his responsibility. Ministers only remain in office for as long as they command the confidence of the PM. Having lost that confidence, he could no longer serve in that role.”

However, Labour insists neither Mrs May nor a senior civil servant should be the arbiter of whether or not there should be a police probe but, rather, it should be the police themselves.

Last night, Tom Watson, the party’s deputy leader, wrote to Mrs May, calling on her "as a matter of urgency" to pass the information gathered during the Whitehall inquiry onto the police "so an appropriate investigation can take place".

Mr Watson said it was "clear" the unauthorised disclosure of secret NSC discussions on the involvement of Chinese tech giant Huawei in the UK's 5G phone network could amount to a breach of the Official Secrets Act.

He told Mrs May: "The lack of action from your Government to refer these matters to the police is putting the Commissioner of the Metropolitan Police in an impossible position.

"She will not be able to investigate without access to the information gathered by your internal inquiry. This information is held by the Cabinet Office.

"It is not for the ministers or civil servants in the Cabinet Office to determine whether the information they have gathered meets the threshold for a criminal investigation. Public interest dictates that it is the police and Crown Prosecution Service that must make this assessment."

Mr Watson has also called for Mr Williamson to forfeit the £17,000 severance payment to which he is entitled on leaving the Government.

The PM’s attempt to draw a line under the Williamson affair was undermined by her Cabinet colleague Jeremy Hunt, who made clear he believed it was up to the police to decide whether or not a criminal investigation should be launched.

Speaking during his Africa tour at a world press freedom day forum in Addis Ababa in Ethiopia, the Foreign Secretary said: “When it comes to issues like whether there should be a police investigation or not, there’s a very, very important principle of our system that those decisions are not made by politicians, they are made independently by police.

“And that has to be the correct way forward in this situation,” he added.

Mr Williamson, who has strongly denied he leaked the NSC information, has himself called for the police to investigate, believing that any criminal inquiry would exonerate him completely.

He has been scathing about his treatment, saying he has been the victim of a “witch-hunt” and “kangaroo court”. The former Secretary of State has declared: “I swear on my children’s lives that I’m innocent.”

A string of senior Conservative MPs have also demanded a full investigation.

One, Sir Desmond Swayne, the former minister, said Mr Williamson had effectively been branded a liar, telling MPs: "Natural justice requires that the evidence is produced so that his reputation can be salvaged or utterly destroyed."

David Lidington, Mrs May’s de facto deputy, was accused of being “casual with the facts” after he responded to an Urgent Question in the Commons on the Williamson case.

He told MPs: "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, co-operate fully should the police themselves consider that an investigation were necessary."

Yet earlier, Cressida Dick, the Met Commissioner, made clear that the force could only investigate the matter of whether Mr Williamson had broken the Official Secrets Act if it received a referral from the Cabinet Office, where Mr Lidington is the lead minister.

She said any official secrets would be owned by the Government, which held 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," explained the Commissioner.

Stewart McDonald for the SNP raised a Point of Order, suggesting Mr Lidington was fully aware that his office would not refer the matter to the police even though he had offered Scotland Yard the Government’s full co-operation with one.

“He must have known when he was on his feet, responding to questions that that was the case,” declared Mr McDonald. “They could not co-operate with a police investigation that the Government has no intention of asking the police to proceed with,” he explained.

Suggesting there was a “degree of being casual with the facts,” the Glasgow MP urged Mr Lidington to return to the House to clarify what the Government’s position was and “what its role might be in getting, what many in this place believe to be pivotal, the police to investigate the entire sorry affair”.

Meanwhile, the Williamson case was raised at Holyrood where, during FMQs, Nicola Sturgeon said it was “reprehensible that there were leaks from the NSC” and that it was a “sign of the complete dysfunction at the heart of the UK Government”.

The First Minister added: “All politicians in Government should recognise the responsibility and the privileges we carry and should not be behaving in the way it Gavin Williamson appears was behaving; for their own selfish political ends."