David Cameron today hailed a "new chapter" in the history of defence co-operation between Britain and France.
After signing two new treaties with French president Nicolas Sarkozy, the Prime Minister said citizens of both countries would be "better protected" as a result.
"Today we open a new chapter in a long history of co-operation on defence and security between Britain and France," Mr Cameron told a press conference at the Anglo-French Summit in London.
One of the treaties would commit the British and French armed forces to work together "more closely than ever before".
The other would increase co-operation on "nuclear safety", he said.
"The result will make our citizens safer, more secure and better protected in the global age of uncertainty in which we now live," Mr Cameron said.
The Prime Minister stressed today's defence agreement was not about weakening sovereignty.
He went on: "This is not about a European army. This is not about sharing our nuclear deterrents.
"Britain and France are and will always remain sovereign nations able to deploy our armed forces independently and in our national interests when we choose to do so."
Mr Cameron pointed out the "vast bulk" of Britain's military operations in recent years had all been undertaken in co-operation with allies.
He said of today's undertaking: "It is about defending our national interest. It is about practical, hard-headed co-operation between two sovereign countries."
He said that by eliminating unnecessary duplication "we can expand our sovereign capability even at a time when resources are tight".
He said the UK and France were "natural partners" and went on: "This is the start of something new, not an end in itself."
The Prime Minister told reporters the agreement would create a new Joint Expeditionary Taskforce - "troops who will train and exercise together".
He also confirmed the UK's new aircraft carriers would be adapted so they could operate with French and American planes, and so that a joint UK-France integrated carrier strike group could be deployed.
Mr Cameron said the two countries would also work together on equipment and capabilities, such as Unmanned Aerial Vehicles and technology needed to counter threats from cyberspace.
On co-operation over the countries' nuclear arsenals, he said a new joint facility would be built.
"While we will always retain an independent nuclear deterrent, it is right we look for efficiencies in the infrastructure required to develop and sustain our separate deterrents," he said.
"Rather than both countries building identical and expensive facilities to ensure the safety of our nuclear weapons, we will build together a joint facility, jointly owned and jointly managed, sharing our knowledge and expertise and saving millions of pounds."
Mr Cameron said: "Britain and France have a shared history through two World Wars. Our brave troops are fighting together every day in Afghanistan.
"But let me finish by saying this is a treaty based on pragmatism, not just sentiment.
"I would like to thank Nicolas for joining me in taking these bold and important steps which I believe will make our sovereign nations safer."
Mr Sarkozy said: "This is a decision which is unprecedented and shows a level of trust and confidence between our two nations which is unequalled in history."
The treaties would deliver "a truly integrated aircraft carrier group", said the French President.
But he rejected claims that they would infringe on either country's sovereignty.
"In France, sovereignty is as touchy an issue as it is in Britain," said Mr Sarkozy.
"But together, we will be stronger, together we will do better, together we will better defend the values that we share."
Mr Cameron said there would need to be "political agreement" between the two countries for a joint taskforce to be deployed.
"Obviously we would only jointly commit a taskforce if we jointly agreed on the mission, but the idea of having a taskforce that trains and works together is an excellent idea because in so many parts of the world we are working together," he said.
"We are working together in Afghanistan, we work together in the Balkans, we support what each other does in the Middle East, so there are many opportunities.
"But in the end this would only happen if there was political agreement for it to happen."
Mr Sarkozy said it would be unlikely that Britain could face the kind of crisis which required the deployment of an aircraft carrier without it affecting France.
"We are not identical, there are many things on which we don't agree and I know that there is the Channel between our two countries," he said.
"However, our values are the same, we share the same values, our interests are fair. All my political life I have argued in favour of rapprochement between London and Paris."
He added: "If you, my British friends, have to face a major crisis, could you imagine France simply sitting there, its arms crossed, saying that it's none of our business?"
Asked about the British rebate from the European Union, Mr Sarkozy said he and Mr Cameron did not agree on "every single minutest detail".
"We've talked about it, we've had discussions, we're agreed on one thing, which is that we have to talk about our problems in order to find solutions together," the French President said.
"I know the Common Agricultural Policy is not the most popular thing here in Britain.
"But let me also share something with you - the British rebate is not something that basically brings the French that much closer to the British.
"But we need to talk about it, as friends, as allies, as people who are responsible, as adults. I can see what Mr Cameron's red lines are and he knows perfectly well what the French government's red lines are."
He said they had agreed to talk about their positions in order to find "common compromises".