France's President Nicolas Sarkozy and British Prime Minister David Cameron have signed two new defence treaties.
Under the agreement, there will be joint training exercises for a new Anglo-French rapid reaction force, at least one aircraft-carrier at sea at all times, and shared facilities for testing nuclear warhead components.
A British centre will develop nuclear testing technology while one in France carries it out and there are plans for a joint army expeditionary force.
Both leaders say the deals will save money but won't affect either country's sovereignty.
"Today we open a new chapter in a long history of co-operation on defence and security between Britain and France," Mr Cameron said, adding that both countries "are, and will always remain, sovereign nations, able to deploy our armed forces independently and in our national interest when we choose to do so."