Theresa May should “wake up” to the seriousness of the leak of sensitive information from the National Security Council and call in Scotland Yard.

The demand for a criminal investigation into how an unprecedented breach of security happened was made by Sir Michael Fallon, the former Defence Secretary, and came as senior Cabinet minister after senior Cabinet minister stepped forward to insist they were not the leaker.

The NSC is a private forum where high-ranking ministers and senior officials from MI5, MI6 and GCHQ can discuss frankly highly sensitive information.

Senior ministerial sources confirmed Sir Mark Sedwill, the Cabinet Secretary and National Security Advisor, had begun a top-level Whitehall probe, enraged about the leak of the Government’s apparent decision to allow the Chinese firm Huawei to supply equipment for the UK’s 5G mobile phone networks.

However, Sir Michael insisted an internal investigation was not enough given the gravity of the security breach.

“This is too serious a matter for that,” he declared. “It does require Scotland Yard to be called in and for the police to be involved. I hope Downing St wake up to that,” insisted the Scot.

He pointed out how if a minister was found to have been responsible, they could face prosecution under the Official Secrets Act.

“It’s extraordinary to think a minister can leak details of the NSC and then think they can get away with it. That’s why a police inquiry now is so important,” added Sir Michael.

Downing St declined to comment on the leak but the Prime Minister’s spokesman reflected her anger by saying: "The Prime Minister is clear: the protection of information on matters of national security is of the highest importance.”

It was suggested a raft of senior ministers present at this week’s NSC had raised security concerns about giving the 5G contract to Huawei, which operates under the governance of a Communist regime.

Among these were Jeremy Hunt, the Foreign Secretary, Sajid Javid, the Home Secretary, and Gavin Williamson, the Defence Secretary; all said to harbour ambitions to succeed Mrs May when she leaves Downing St.

One Whitehall insider claimed the leak was "evidently briefed to make a leadership candidate look tough on China".

But, one by one, ministers came forward to decry the security breach and insist they were not to blame.

Mr Hunt, speaking at a press gallery lunch at Westminster, described it as “utterly appalling”. When asked directly if he or any of his staff had leaked the NSC information, the Foreign Secretary replied firmly: “No.”

Mr Javid said it was "completely unacceptable" for any Government minister to "share sensitive information that cannot be out in the public domain" and if it happened, it should "absolutely be looked at".

Mr Williamson made clear: “Neither I nor any of my team have divulged information from the National Security Council."

Such was the concern about the leak that Labour was successfully granted an Urgent Question in the Commons.

Jo Platt, the Shadow Cabinet Office Minister, who called for a leak inquiry, said if it was found a minister had been responsible, then they were not fit to be in Cabinet or to become prime minister.

"Indeed, if the leak was for an advantage in a Tory leadership race that would be truly shocking. Critical issues of national security should be handled with utmost care, not used as political ammunition in a Tory Party civil war,” insisted the Greater Manchester MP.

The SNP’s Martin Docherty-Hughes, who sits on the Commons Defence Committee, also denounced what he described as a “shocking breach of protocol at the very highest levels”.

The West Dunbartonshire MP added: “It is unacceptable that the NSC has been shamefully turned into a tool for Tory leadership contenders.

“A full and thorough inquiry must get to the bottom of this, and if needed, a criminal inquiry, so those found to be responsible are properly held to account."