France and Britain have announced a new level of cooperation on their nuclear deterrents, signalling a united European response to growing regional threats and uncertainty over continued U.S. defence support.

The agreement, revealed at the conclusion of French President Emmanuel Macron’s three-day state visit to the UK, aims to reset Franco-British relations following years of strain since Brexit.

British Prime Minister Keir Starmer, speaking at a joint press conference with Macron, said the two nations had signed the Northwood Declaration, marking the first time their independent nuclear forces will be formally coordinated.

“From today, any severe threat to Europe will be met with a joint response from our two countries,” Starmer said. “This is the clearest signal yet of the strength and significance of our relationship.”

The move comes amid growing unease in Europe over U.S. reliability, particularly after President Donald Trump questioned NATO burden-sharing and expressed warmer ties with Russia. Europe has long relied on U.S. nuclear protection as a pillar of trans-Atlantic defence, raising concerns as Washington’s commitment appears less certain.

Macron said the two countries had set up an oversight body to manage the enhanced cooperation and emphasised the symbolic and strategic value of the announcement.

“This decision does not rule out coordination of our deterrents,” Macron said. “It’s a message our allies and rivals alike must take seriously.”

He added that the agreement was separate from ongoing discussions about a European 'coalition of the willing' to support Ukraine should a ceasefire with Russia occur.

While both Britain and France will retain full independence in their nuclear decision-making and maintain strategic ambiguity, the new alignment strengthens Europe’s security at a time of geopolitical instability.

Macron has previously floated the idea of extending France’s nuclear umbrella to protect other European partners.

The United States maintains nuclear weapons and tens of thousands of troops across Europe, giving it unmatched military capabilities, but doubts persist over whether this presence will remain unchanged.

France currently spends about €5.6 billion ($6.54 billion) annually to maintain its arsenal of 290 nuclear weapons, the fourth largest in the world. Britain’s nuclear programme is officially independent but relies on U.S. missile technology and logistical support.

Starmer called the agreement “truly historic,” underscoring its importance in ensuring European security amid a shifting global landscape.

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