The Lancaster House defence treaties were signed by Britain and France in 2010. They state that a threat to the "vital interests" of one country is a threat to the other. The two countries are Europe's only nuclear-armed powers and its only permanent members of the UN Security Council.
In July 2025, building on the treaties, Britain and France for the first time agreed to "co-ordinate" the use of their nuclear weapons, declaring "there is no extreme threat to Europe that would not prompt a response by both nations."
The Franco-British defence relationship was strained by the AUKUS submarine deal of 2021, which cancelled a Franco-Australian submarine contract. By mid-2025 the relationship had recovered; Lord Ricketts, a former British ambassador to Paris, described it as "certainly the best it's been since Brexit."
Ever get the feeling that the world's on tape and one of the reels is missing?