The UK and France have agreed to co-ordinate their nuclear deterrents, Sir Keir Starmer said, in a joint show of force against adversaries like Russia.
The Prime Minister said there was “no greater demonstration” of the importance of the UK-France relationship than the new initiative.
The Northwood Declaration signed by Sir Keir and French President Emmanuel Macron, means that Europe’s two sovereign nuclear powers would mobilise their deterrent together if threatened.
The Franco-British move is designed to act as a warning to Russia.
It comes as US President Donald Trump has called on European allies to step up their contribution to Nato, and as the US tilts its influence away from Europe towards the Pacific.
Sir Keir outlined a series of joint military ventures the UK and France would undertake together as he and the French president visited the UK’s military headquarters, Northwood, in north-west London.
The Prime Minister said: “Now as Europe’s only nuclear powers and as leaders in Nato, we play a vital role in preserving the peace and security on this continent.
“So today, we’ve updated the historic Lancaster House treaty to protect our people and our way of work. This is a major modernisation.
“We are overhauling combined joint expeditionary force to make it five times larger, 50,000 troops strong, able to act across every domain.
“But with going further, this morning, we signed the Northwood Declaration, confirming for the first time that we are co-ordinating our independent nuclear deterrents.
“From today, our adversaries will know that any extreme threat to this continent would prompt a response from our two nations.”
He said of the Northwood Declaration: “There is no greater demonstration of the importance of this relationship, and while we stand together for our collective defence, we must also deliver a defence dividend for working people so we’ve agreed a deeper industrial partnership today to bring our defence industries closer than ever before.”
Alongside the new nuclear pact, and refreshed joint force, France and Britain have both pledged to replenish their stocks of Storm Shadow missiles.
Both countries have gifted the long-range weapons to Ukraine, allowing Kyiv to strike deep into Russian territory.
The two nations also plan to work more closely on developing a successor Storm Shadow.
