A UK centre will develop nuclear testing technology while one in France carries it out and there are plans for a joint army expeditionary force.

After signing the treaties with Nicolas Sarkozy, the UK PM said it would make citizens safer and would save money. But Labour suggested it could limit the UK's ability to act independently.

After he and French President Nicolas Sarkozy signed the two treaties, Mr Cameron said: "Today we open a new chapter in a long history of co-operation on defence and security between Britain and France." He said it was not about a European army or about sharing nuclear weapons.

"Britain and France 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."

But Mr Cameron said the vast bulk of Britain's military operations over the past few decades had been carried out with allies and said co-operating on testing nuclear warheads would save millions of pounds.