BRITAIN’S top security official has launched an inquiry into the leak of sensitive details from secret talks within the National Security Council, The Herald has been told.

The development came as Jeremy Hunt, the Foreign Secretary, who attended this week’s NSC meeting, emphatically denied he or his staff were responsible for the unprecedented breach of confidence.

Sir Mark Sedwill, the Cabinet Secretary and the National Security Advisor, is said to have been enraged by the leak about the Government’s apparent decision to allow the Chinese firm Huawei to supply equipment for the UK’s 5G mobile phone networks.

A senior ministerial source confirmed that Sir Mark had begun a top-level inquiry. It is believed the launch was accompanied by a “strongly-worded” letter being sent to attendees of NSC meetings, which not only include senior ministers but also security and armed forces officials.

The Cabinet Office made clear it would not comment on any leak inquiry.

Speaking at a Westminster press gallery lunch, Mr Hunt described the breach of security as “utterly appalling”.

He told reporters: “It’s a really bad thing for decision-making in Government. I, as everyone here knows, have never leaked confidential Cabinet discussions and I never will…It is a very bad day for our democratic processes when that kind of thing happens.”

Asked directly if he or his staff were involved in the leaking, the Secretary of State replied emphatically: “No.”

Earlier, the Government made clear a criminal investigation into the leak could not be ruled out as MPs said that any minister who was responsible for the leaking of the NSC details should be sacked immediately.

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

He declined to say whether a leak inquiry had been launched, telling reporters at a regular Westminster media briefing: "I don't think you would expect me to comment upon leaks or questions about leak inquiries."

The leaking of discussions from the NSC has caused great alarm across Whitehall.

As well as Cabinet ministers, the council is attended by security chiefs from MI5, MI6 and GCHQ. It has been suggested intelligence chiefs are furious that what has been a traditional iron-clad confidentiality of the weekly meeting, in which classified information is shared, has been broken.

Tory grandee Sir Nicholas Soames, the former Armed Forces Minister, has called for a criminal inquiry. He suggested the leak would “cause our friends and allies to wonder if we can be considered reliable; whoever is responsible should be dismissed from the Queen’s service”.

The leak has been linked to the race to succeed the Prime Minister when she steps down, which, it is thought by many, could be after the elections in May.

One source said it was briefed to "make a leadership candidate look tough on China”.

Those present at the NSC who expressed concern about the Government’s apparent decision to allow the Chinese firm Huawei to supply equipment for the UK’s 5G mobile phone networks included Mr Hunt, Sajid Javid, the Home Secretary, Gavin Williamson, the Defence Secretay, Liam Fox, the International Trade Secretary, and Penny Mordaunt, the International Development Secretary.

As the clamour grew for a Whitehall leak inquiry, MPs from both sides of the House made clear that if a minister were responsible, then he or she should be sacked.

Dominic Grieve, the chairman of the Parliamentary Intelligence and Security Committee, described the NSC leak as "deeply worrying".

The Conservative backbencher claimed collective Cabinet responsibility appeared to be breaking down and that if any minister were found to be responsible, he or she should be sacked immediately.

"The principle that what is discussed at the NSC is kept totally confidential is really important," Mr Grieve declared.

"That that should have been breached in this fashion - and I can't think of any occasion where it has happened before - is deeply worrying.

"If it turned out that it was a member of the Cabinet - or indeed a minster who was attending the NSC - they should be sacked immediately," insisted the former Attorney General.

He acknowledged the leak could be linked to the belief among ministers that a Tory leadership contest could be imminent.

"There has certainly been some posturing around on a whole range of issues. I can't pretend that there aren't people who appear to be preparing themselves for leadership bids. None of this is in the national interest,” added the Buckinghamshire MP.

Asking an Urgent Question in the Commons on the Huawei controversy, Jo Platt, the Shadow Cabinet Office Minister, told MPs: "What a mess we're in. The only reason we know the decision to green-light Huawei is from an apparent ministerial leak of a meeting of the NSC, which has only served to raise public concern whilst undermining the integrity of our security agencies.

"If a minister did leak the information, they are not fit to serve in the Cabinet and are certainly not fit to be Prime Minister,” declared Ms Platt.

"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.

"A full leak inquiry should be undertaken and, if identified, the individual should immediately resign or be removed from their position,” added the Greater Manchester MP.

Jeremy Wright, the Digital Secretary, made clear NSC discussions "should be confidential" and told MPs: "We cannot exclude the possibility of a criminal investigation."

He explained: "Officials, including the security and intelligence agencies, need to feel that they can give advice to ministers which ministers will treat seriously and keep private. If they do not feel that, they will not give us that advice and Government will be worse as a result.”

In what appeared to be a signal that the Government had or was about to launch a leak inquiry, he added: "That is why this is serious and that is why the Government intends to treat it seriously."

Ms Platt also highlighted concerns raised by security services about the wisdom of granting a 5G contract to Huawei but Mr Wright replied: "There has not been a final decision made on this subject and so she is wrong to describe matters in the way that she has."

The Secretary of State said concerns raised by the oversight board about Huawei were linked to "technical deficiencies" of equipment it supplied and not concerns about the "manipulation of that equipment by foreign powers", adding: "They are nonetheless serious and they will be addressed."

He explained that the objective of a Government review was to ensure the "security of the supply network, regardless of who the supplier is, is improved", noting: "It would be wrong to focus entirely on Huawei or Chinese equipment.

"However, it is worth recognising that Chinese equipment and indeed Huawei equipment is indeed prevalent across the world, not just in the United Kingdom; there is a good deal of Huawei equipment already in the UK networks, so we're not talking about beginning from a standing start.

"But it reinforces in my view the need to ensure this review of the supply chain is broadly based as it is to ensure we address the security of the network, regardless of where the equipment comes from."

Mr Wright stressed "full account" was taken of what the security and intelligence agencies had advised the Government on the subject, adding they would continue to "take seriously what they tell us".

In Glasgow at the CyberUK conference, David Lidington, the PM’s de facto deputy, also insisted the UK took the security of the country’s telecommunications networks extremely seriously.

“We have rigorous and tested procedures in place today to manage risks to national security,” he declared.

The Cabinet Office Minister said the next generation networks like 5G raised security risks as well as opportunities for prosperity.

“That is why the Government commissioned a comprehensive review of the telecommunications supply chain. This is a serious study, it is based on evidence and expertise not suspicion or speculation.

"The Government is committed to strengthening significantly this country's framework for telecoms. And we will take whatever steps are necessary to ensure the secure role out of 5G and full fibre network. We will not countenance high-risk vendors in those parts of the UK's 5G network that perform critical security functions.”

Mr Lidington pointed out the Government approach was not about one company or even one country; it was about ensuring stronger cyber security across telecoms, greater resilience in telecoms networks and more diversity in the supply chain.

“We shall want to work with international partners to develop a common global approach to improving telecoms security standards.

"As with any other review, certainly with this complexity and scale, the decisions will be announced in due course and to parliament first."

He noted how said the UK's current strategy lasted until 2021, adding: “We need now to consider our vision beyond that date and how we sustain long-term change."