Britain and France will join together to use nuclear weapons against any “extreme threats” towards Europe, Sir Keir Starmer and Emmanuel Macron have declared.
A newly-signed declaration by the leaders will state that the two countries’ nuclear deterrents are independent but can be co-ordinated in a joint attack on Europe’s enemies.
Officials warned that any adversary threatening the interests of Britain and France would be confronted by the nuclear arsenals of both nations simultaneously.
It came as the pair also agreed to ramp up orders of deadly Storm Shadow missiles to bolster Europe’s defences. In a move that will sustain 1,300 jobs in the UK, Britain and France agreed an “Entente Industrielle” strengthening coordination on air-to-air missiles, AI and directed energy weapons such as lasers.
Sir Keir said: “From war in Europe, to new nuclear risks and daily cyber-attacks – the threats we face are multiplying.
“As close partners and NATO allies, the UK and France have a deep history of defence collaboration and today’s agreements take our partnership to the next level.
“We stand ready to use our shared might to advance our joint capabilities - equipping us for the decades to come while supporting thousands of UK jobs and keeping our people safe.”
And defence secretary John Healey added: “The UK and France are stepping up together to meet today’s threats and tomorrow’s challenges.
“This partnership strengthens our leadership in Europe, ensures continued support for Ukraine, and sends a clear signal to our adversaries that we stand stronger, together.”

As well as agreeing to purchase more Storm Shadow missiles, Britain and France will work together on the design for a long-term replacement for the highly-lethal long-range missile.
The “Entente Industrielle” includes an agreement to replace Storm Shadow with a new generation of “deep strike, anti-ship missiles”.
It also commits Britain and France to upgrading production lines of existing Storm Shadows to bolder national stockpiles.
The countries will jointly develop the next generation of air-to-air missiles for the RAF, as well as working together on high-tech anti-drone equipment such as microwave weapons and jammers.
The efforts will be underpinned by collaboration on AI to make missiles and drones more lethal, including through allowing synchronised strikes.
Officials said British and French troops will work closer than ever amid the growing threat to European security from Russia. Sir Keir’s efforts to lead a so-called Coalition of the Willing peacekeeping force in Ukraine have stalled as ceasefire talks between Volodymyr Zelensky and Vladimir Putin remain on hold.
Donald Trump is increasingly seeking to pile pressure on Putin to end the war, having initially lashed out at Mr Zelensky and blamed him for the ongoing invasion.
In a sign of his growing frustration at Moscow, the US president on Wednesday said the war “should have never happened”, that it “should end” but that the US is getting “a lot of bulls*** thrown at us by Putin”.
The defence deal comes ahead of the final day of Mr Macron’s state visit to the UK, with the French president and Sir Keir also expected to strike a deal aimed at cutting the number of migrants crossing the Channel in small boats.
At a Downing Street meeting on Wednesday, the pair discussed the “need for a new deterrent” to tackle the crisis.
A full UK-France summit on Thursday will involve ministerial teams from both nations in the discussions.
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