Sir Keir Starmer is to warn Nato today that it must become a more European-based defence force and reduce its reliance on the US.
In his first major security speech since Donald Trump threatened to annex Greenland, the Prime Minister is expected to say Nato must change fundamentally.
His speech to the Munich Security Conference in Germany will suggest the UK must reintegrate with Europe for security and economic reasons, No10 sources said.
The PM, who rallied to Denmark's side when President Trump vowed to seize the Arctic island, is expected to admit that the UK and other European nations have been overdependent on America.
He will make a case for Britain and her European allies reshaping their relationship with the US based upon interdependence, rather than dependence.
Sir Keir is expected to say: 'We are not the Britain of the Brexit years any more, because we know that, in dangerous times, we would not take control by turning inward. We would surrender it. I won't let that happen. 'There is no British security without Europe, and no European security without Britain. That is the lesson of history - and it is today's reality too. 'I'm talking about a vision of European security and greater European autonomy. That does not herald US withdrawal but answers the call for more burden-sharing. 'British companies already account for over a quarter of the continent's defence industrial base. We want to bring our leadership in defence, tech and AI together with Europe to multiply our strengths and build a shared industrial base across Europe which can turbo-charge defence production.'
Weeks ago, the future of Nato was threatened when President Trump refused to rule out taking military action against another Nato member to occupy Greenland.
The PM will add that the US's national security position is 'evolving' - a reference to President Trump's focus on security in the Americas and the Pacific.
That threat to Denmark, a US ally that sent troops to join US-led forces in Afghanistan, appears to have been a wake-up call for Europe.
A year ago at the same conference, US Vice-President JD Vance shocked delegates by attacking European values and attitudes to free speech.
German Chancellor Friedrich Merz told delegates yesterday there was a 'rift' in Europe's relationship with the US as it emerged that Germany was holding secret talks with Emmanuel Macron of France about sharing nuclear capability.
Last night, Shadow Foreign Secretary Priti Patel said: 'Britain is uniquely place to help bring the US and Europe together.
'We must not be overdependent on America, but neither should we offer Europe a blank cheque.'