NATO members agreed on Sunday to the five per cent of gross domestic product target, a big increase in their defence spending, in response to US President Donald Trump's insistence that Europe take more responsibility for its own security.
On the eve of his departure to a NATO summit in The Hague, Starmer for the first time publicly said Britain would boost spending on defence and broader security measures to the highest level since the Cold War.
The government will also on Tuesday publish a new National Security Strategy that will recommend increasing the protection of sectors such as transport, energy networks, and digital infrastructure from sabotage and cyber-attacks.
"We must navigate this era of radical uncertainty with agility, speed and a clear-eyed sense of the national interest," Starmer said in a statement.
Britain already said this year it would increase defence spending to 2.5 per cent of GDP by 2027 from 2.3 per cent last year.
But the new pledge raises questions about where the money will come from and the depth of curbs in other public services required to meet the target.
The two-day NATO meeting in The Hague is aimed at satisfying Trump with a big new defence spending goal, but it now risks being dominated by his decision to join Israel's attacks on Iran's nuclear plants.
Downing Street said Britain would commit to spending 3.5 per cent of national income on traditional defence and a further 1.5 per cent on broader security.
Under the new NATO target, Downing Street said Britain expects to increase overall defence spending to at least 4.1 per cent of GDP by 2027.
The government will publish the new National Security Strategy, drawing together six reviews since Labour won the general election last year, including inputs from the long-awaited audit of relations with China.
But in a surprise decision, Downing Street said the China audit would not be published separately.
Downing Street said the strategy would recommend that Britain reduce its dependency on other nations in key areas and this will mean being "more unapologetic and systematic in pursuit of our national interests".
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