Former civil servant Simon McDonald has been accused of "trading away our strengths" after suggesting the UK should hand the EU its security council seat.
In his new book, 'Beyond Britannia: Reshaping UK Foreign Policy', McDonald also says the UK should get rid of its nuclear deterrent.
McDonald served as the Permanent Under Secretary and Head of the Diplomatic Service at the Foreign & Commonwealth Office from August 2015 to August 2020.
He now sits as a crossbench peer in the House of Lords.
Simon McDonald has been accused of 'trading away our strengths'
PA
Deputy Conservative Party Chairman Lee Anderson told GB News the former Foreign Office Permanent Secretary's opinion is "not shared by the vast majority" of Britain.
He said: "He is entitled to his opinion, fortunately, his opinion is not shared by the vast majority of the British public and thank goodness for that."
Meanwhile, MP Tom Tugendhadt wrote on X: "Trading away our strengths is hardly a recipe for success."
In a previous interview with LBC, McDonald described nuclear weapons as "the most dreadful weapons ever invented".
Three years ago, McDonald - who was the top Foreign Office civil servant at the time - admitted telling colleagues that he voted to remain in the EU during the 2016 referendum.
This is despite the mandate for civil servants to remain politically impartial and never allow "personal political views to determine any advice you give or your actions".
But it does not prohibit holding a view that opposes that held by the government of the day.
Speaking about his decision to tell colleagues he voted to remain, McDonald said he was "trying to maintain credibility".
He added that he was "trying to convey a message to a group of people, most of whom I felt had voted to remain in the EU, that their personal feelings were beside the professional point".
The summary of McDonald's book argues that the UK's "significant soft-power strengths can be harnessed to expand our international influence".
It says: "Such a shift will only be possible, he says, if we first acknowledge the challenges of Brexit and the need to reduce our unrealistic hard-power ambitions.
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"Excellence in areas that other countries care about will keep the UK internationally relevant in the second half of the century in a way that nostalgia for a lost pre-eminence will not."
The publisher's synopsis warns: "For too long successive governments have shied away from acknowledging uncomfortable truths about the decline of Britain's military capabilities".
Beyond Britannia: Reshaping UK Foreign Policy was published last month.