Uncovered documents show the Labour leader advocated for ripping up the defensive alliance while in his late 20s. He suggested the UK and other Western countries should be “abandoning weapons and strategies of genocide”.
Since taking over as Labour leader, the Holborn and St Pancras MP has sought to position himself as tough on national security in the face of Russian aggression.
Looking to paint himself in a different light to the pacifist views of his predecessor Jeremy Corbyn, he proudly declared Labour supporter of Nato.
When at the helm, Mr Corbyn refused to rule out scrapping Trident if he won the 2019 general election.
Distancing himself earlier this month, Sir Keir wrote in a newspaper article: “To condemn Nato is to condemn the guarantee of democracy and security it brings, and which our allies in eastern and central Europe are relying on, as the sabre-rattling from Moscow grows ever louder.”
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He added: “There is no equivalence between a defensive alliance that has never provoked conflict and those who would inflict the appalling cost of war on to others.”
But documents seen by Express.co.uk show that Sir Keir once held a very different stance.
While Secretary of the Haldane Society of Socialist Lawyers, the group passed a resolution calling on “Britain and other capitalist countries” to disarm.
It added that the Labour Party should “adopt a non-nuclear, non-aligned defence policy as the precondition for the preservation and extension of human rights”.
The motion essentially backed scrapping Britain’s nuclear deterrent and leaving the defence alliance.
The group claimed the policy was “key to victory at the next general election”.
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In 1992, at the next national vote, the Conservatives won a fourth decisive victory over Labour.
The motion as passed was printed in the issue No9 of the Socialist Lawyer publication in autumn 1989.
A Tory source told Express.co.uk: “Starmer is now at pains to break with Corbyn’s leadership, who he loyally served under for four years.
“But as a young lawyer he was in fact in perfect sync with Corbyn’s views, with his Society of Socialist Lawyers demanding the UK scrap Trident and leave Nato.”
The Labour Party has been contacted for comment.
On February 10 Sir Keir met Nato’s secretary-general Jens Stoltenberg in Brussels as part of a bid to change Labour’s image on national security.
He said the party was committed to the defence bloc.
“This is my first chance as leader of the Labour Party to come here to Nato headquarters to have the meeting with the secretary-general and to deliver a very important message for our party and for our country, which is that the Labour Party support for Nato is unshakeable,” he said at the time.