Senior MoD figures thought Gavin Williamson caused national security leak

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Penny Mordaunt was forced to deal with a security leak during her time as defence secretary that the department believed came from her predecessor, Gavin Williamson, amid fears the information put “our people’s lives at risk”.

Three sources told the Guardian that the breach was deemed so serious that Mordaunt was prepared to seek a D notice to warn any media considering publishing the information that it risked endangering Britain’s national security.

But Williamson, who denied leaking national security information that saw him ejected from the cabinet in 2019, said he had nothing to do with the second serious alleged disclosure.

A former government insider said it was believed by senior figures in the Ministry of Defence at the time that the leak “could only have come from Gavin” and that “our people’s lives were put at risk by it”.

They declined to discuss the details of the alleged leak, for the same security reasons, but said the issue was of significant concern during Mordaunt’s short time as defence secretary between May and July of 2019.

Another source said officials believed “100%” the leak “came directly from Williamson” and that Mark Sedwill, the then-cabinet secretary and national security adviser, was told so by the security services.

The third source said Mordaunt successfully ensured the leak was contained and that the move had never previously been publicly disclosed.

A spokesperson for Williamson told the Guardian the accusation he was the culprit behind a second major leak was “categorically untrue” and added: “He has no knowledge of this or any involvement with it.”

D notices can be issued by the secretary of the Defence and Security Media Advisory Committee to request for media not to report certain information on grounds it could prejudice the UK’s national security.

If deemed necessary, the secretary of the committee, currently a former army officer, will issue a letter outlining specific concerns after consultation with the government department concerned. It is not, however, mandatory to follow the advice.

A government spokesperson said: “We do not comment on matters of national security or alleged leak investigations.” It was also disputed that Williamson had been formally suspected or accused inside Whitehall of being responsible for a second significant leak. Mordaunt, who is now leader of the Commons, declined to comment.

Williamson had been sacked by former prime minister, Theresa May, in May 2019 for leaking details from a contentious discussion at the national security council about how to handle Chinese telephone equipment maker Huawei to the Daily Telegraph. He denied being the source of that leak.

Williamson dramatically resigned on Tuesday night after a string of complaints about bullying and threatening conduct by him when he was a cabinet minister.

A former official at the MoD made an official complaint on Tuesday after the Guardian revealed that Williamson told them to “slit your throat”, in what they felt was a sustained campaign of bullying.

It is not clear if the prime minister, Rishi Sunak, was aware of the prior security concern when he appointed Williamson as a minister of state in the Cabinet Office.

His short-lived responsibilities included the UK’s global Conflict Stability and Security Fund and the Government Communications Service, to which departmental press officers belong.

Several ministers, advisers and officials believed that Williamson was leaky and tried to brief stories to the press that contained sensitive classified material – although Williamson himself denied passing on information about Huawei.

A former Downing Street staffer told the Guardian that there was “substantial information” that proved Williamson was the leaker, and not all of it was put into the public domain at the time. “We didn’t want to reveal our methods,” the ex-staffer said.

There was surprise within Downing Street that Williamson so strongly denied being the source – at the time, it was reported he swore on his children’s lives that this was not the case – when No 10 believed it had all the evidence it needed to prove his involvement.

“The only explanation for his denials at the time were that he wanted to stage a comeback,” the former staffer said. Williamson was reappointed as education secretary by Boris Johnson, but sacked in a reshuffle before being brought back by Sunak.