Boris Johnson has been accused of adopting a "more casual" approach to Britain’s national security, as MPs warned the Whitehall machine was unable to cope with more than one major crisis at a time.
In the report, 'The UK's National Security Machinery', the committee said the lack of preparation for the withdrawal of international forces from Afghanistan could "only be described as a systemic failure".
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It added that reforms to the National Security Council (NSC) meant the Prime Minister would slash the time he spent leading its meetings by around two-thirds.
The NSC was originally established in 2010 by David Cameron with weekly meetings bringing together senior ministers and defence and intelligence chiefs chaired by the Prime Minister.
However, the committee said a review by National Security Adviser Sir Stephen Lovegrove into its operation would in the future result in Mr Johnson chairing only about half its meetings.
"Most concerningly, the new model for the NSC risks becoming a halfway house: it is neither a slower-paced forum for tackling the most fundamental questions facing UK national security, nor a weekly meeting of senior ministers – convened and brokered by the Prime Minister – to tackle the most pressing issues," the report said.
"It is the Prime Minister's personal investment of time and authority that lends credibility to the NSC and its cross-government structures.
"Yet under the new system, the Prime Minister will spend roughly 65% less time in NSC meetings than under the previous practice of weekly meetings when Parliament is in session.
"In our initial assessment, therefore, this is a retrograde step that suggests a more casual approach to national security."
The report said there was already a "troubling lack of clarity" about the role and remit of the NSC.
When the COVID-19 crisis broke, the structures of the council were said to have been abandoned in favour of "ad-hoc arrangements and improvisation" – a move the committee said was "a serious mistake".
Once the Afghan crisis broke over the summer, the Government was "unable to prepare for and respond to two national security crises simultaneously", the report added.
With one expert witness warning of a one-in-six chance of an "existential catastrophe" over the next 100 years – from extreme climate change scenarios to nuclear conflict – the committee said there was an urgent need for reform.
The committee chair, former foreign secretary Dame Margaret Beckett, said: "I pay tribute to the medical and military personnel and the civil servants who have worked so hard to respond to these two most recent crises.
"But their brave efforts cannot mask the fundamental need for the centre of government to get a grip on national security planning."
The report called for routine engagement with devolved authorities in Scotland, Wales and Northern Ireland, a role in funding allocation for security priorities and a return of the ministerial committee for managing risks and resilience.
It also recommended external, diverse input into NSC discussions to avoid 'groupthink'.
A Government spokesman said the national security machinery was being strengthened following the Government's Integrated Review of foreign and defence policy earlier this year.
"The Prime Minister is clear that the first duty of the Government is to provide security for the people of the UK, which he is absolutely committed to delivering," the spokesman said.
"The Prime Minister and his Government have just agreed a unique trilateral defence arrangement with Australia and the United States.
"The national security machinery is being strengthened in response to the Integrated Review as well as events such as COVID and Afghanistan as necessary."