THERESA May has sacked Gavin Williamson as defence secretary, accusing him of leaking secret National Security Council discussions.

The former minister, who is now the 42nd person to have left May’s Government, denied that he was responsible for telling press about plans to allow Chinese state-owned firm Huawei to supply technology for the next-generation 5G network.

Labour and the SNP have both called for a criminal inquiry.

The National Security Council (NSC) is one of the most secret meetings in Whitehall. As well as senior cabinet ministers, it is attended by the heads of MI5, MI6 and GCHQ.

When information about the Huawei decision was leaked to the Daily Telegraph a week ago, the intelligence chiefs were furious, and Sir Mark Sedwill, head of the civil service, opened a major inquiry.

Williamson had been a key suspect, though he strongly and repeatedly denied the allegations.

In a letter sent to him yesterday after a meeting in her office, May said the denial was not believable. She also accused him of not engaging fully with Sedwill’s probe,

“It has been conducted fairly, with the full co-operation of other NSC attendees. They have answered all questions, engaged properly, provided as much information as possible to assist with the investigation, and encouraged their staff to do the same. Your conduct has not been of the same standard as others,” the Prime Minister wrote.

“In our meeting, I put to you the latest information from the investigation, which provides compelling evidence suggesting your responsibility for the unauthorised disclosure. No other, credible version of events to explain this leak has been identified.”

May said that it was with “great sadness” that she could “no longer have full confidence” in Williamson as Defence Secretary.

Replying to the Tory leader, Williamson denied he was responsible: “I am sorry that you feel recent leaks from the National Security Council originated in my department. I emphatically believe this was not the case. 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.”

Penny Mordaunt will replace Williamson in the Ministry of Defence, with Rory Stewart replacing her at the Department for International Development.

Labour’s Tom Watson tweeted: “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.”

The SNP’s Stewart McDonald agreed. He said Williamson had “let down a great many people, none more than those in uniform who protect us”.

He added: “It is completely unacceptable that 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 that Mr Williamson has broken the law, and if so, the matter must be passed to the police for a full criminal investigation.

“Given the gravity of the allegations, Theresa May must immediately suspend Mr Williamson from the Conservative Party. Mr Williamson should also consider whether he can continue as an MP.”

According to the Telegraph, the NSC agreed in principle to allow Chinese state telecoms firm Huawei to supply “non-core” technology in future 5G networks.

According to the newspaper’s report, Home Secretary Sajid Javid, Foreign Secretary Jeremy Hunt, International Trade Secretary Liam Fox, Mordaunt and Williamson all expressed concern about the decision.

The US said on Monday it would review intelligence sharing with the UK over the decision.

The Government says a final decision has not yet been made.