Video: David Cameron and Nicolas Sarkozy set out joint defence strategy Reuters

Britain and France today signed a landmark 50-year treaty on defence and security that envisages the joint use of aircraft carriers, a 10,000-strong joint expeditionary force and unprecedented new levels of co-operation over nuclear missiles.

The deal, signed in London by David Cameron and the French president, Nicolas Sarkozy, has in part been forced on the two countries as they struggle with tightening defence budgets, but also reflects a level of mutual trust not seen for decades.

At a joint press conference at Lancaster House, Cameron repeatedly stressed that the agreement strengthened British sovereignty as he said it opened a new chapter in Anglo-French relations.

Seeking to defend himself from a Eurosceptic assault, with one Tory MP describing the French as "duplicitous", Cameron stressed the treaties would not weaken British sovereignty and did not amount to a sharing of the UK's nuclear deterrent.

He said: "Britain and France will be sovereign nations able to deploy our forces independently and in our national interest when we choose to do so.

"The two largest defence budgets in Europe are recognising that if we come together and work together we increase not just our joint capacity, but crucially we increase our own individual sovereign capacity so that we can do more things alone as well as together."

Nicolas Sarkozy and David Cameron joke after signing the Anglo-French defence treaty today. Photograph: Leon Neal Pool/EPA

Sarkozy hailed the agreement in even more enthusiastic terms: "This is a decision that is unprecedented and it shows a level of co-operation and confidence between our two nations that is unique in history."

Cameron stressed that Britain would retain he ability to fight alone, but pointed out that British troops had in practice only operated independently twice in the past 30 years – in Sierra Leone and in the Falklands. The bulk of UK military activity was undertaken in co-operation with allies, and he said anything that strengthened overall UK military capacity would be welcomed by Brtain's international partners, including the US.

He described the agreement as a "practical, hard-headed agreement between two sovereign countries". The prime minister said the agreement would reduce development costs, eliminate duplication and align research programmes.

Britain and France were natural partners as the third and fourth largest forces in the world, he said, calling France a logical sensible and practical partner. "It is the start of something new," Cameron said, adding: "The treaty is based on pragmatism, not just sentiment."

Cameron disclosed that the two treaties would:

• Create a joint expeditionary task force in which around 5,000 UK and French troops from all three services will train and exercise together from 2011. The force will be able to launch high-intensity peacekeeping, rescue or combat missions.

• Co-operate on an integrated strike force ensuring aircraft can operate off both the planned British carrier and the French carrier Charles de Gaulle. Operations would start with training missions, and would eventually provide cover when one nation's carrier was in dock for maintenance. Britain will modify its future aircraft carrier to make it compatible with French aircraft.

There will also be new co-operation on procurement, including a joint common support plan for the future fleets of A400m transport aircraft being brought by both countries. The two sides also agreed to work together over the next 10 years on unmanned drone aircraft, as well as on nuclear submarine technology.

In a separate agreement that Cameron said would save hundreds of millions of pounds, the two countries agreed to co-operate on nuclear safety by building a jointly run facility at Valduc, near Dijon, that will model performance of UK and French nuclear warheads and materials in co-operation with a joint Technology Development Centre at Aldermaston.

The French laboratory will host British defence scientists to carry out tests on their country's nuclear warhead stockpile, soon to be cut to 120.

In return, French officials will be stationed at the UK facility to work on nuclear test technology.

Cameron said "there was no more sensitive issue on which two countries could co-operate than nuclear technology".

Further details will be given by the defence secretary this afternoon in a Commons statement demanded by Labour and granted by the Speaker.

Sarkozy insisted France was not abandoning its national sovereignty, but throughout struck a more enthusiastic, visionary tone about the agreement than Cameron.

He said: "In France sovereignty is as touchy an issue as it is in Britain. Together we will be stronger. Together we will do better. Together we will better protect our values. We cannot solve problems of 21st century with the ideas of the 20th century."

He added: "It is fashionable to say that Europe lacks strategic vision, but Britain and France have pooled their sovereignty."

Sarkozy said he could not envisage a crisis so grave that France and Britain would not be willing to undertake joint military actions. He said if Britain needed to send out an aircraft carrier, "France would not sit there with its arms crossed saying 'it is none of our business'."

Sarkozy stressed that Britain and France had shared values and said he had always favoured closer co-operation between the two countries. "All my political life I have argued in favour of a rapprochement between London and Paris. It has been a constant commitment since I got involved in politics.

"Sovereignty does not mean isolation. When you are isolated you are no longer sovereign. You are exposed."

In a sign of the political debate ahead, the Conservative former defence select committee member Bernard Jenkin warned: "Tactical co-operation, yes. Strategic fusion, no. We need to recognise that France has never and is never likely to share the same strategic priorities as the UK.

"There is a long track record of duplicity on the French part. When it comes to dealing with allies, we should never be under any illusion. The French act in what they see as their strategic interests."

Jenkin questioned whether France would make an aircraft carrier available for an operation like the relief of the Falklands, and warned that the US might "cut off" intelligence co-operation with the UK if it appeared secrets were being shared with Paris.

But Lord Ashdown, the former Liberal Democrat leader, told the BBC: "The French believe we are duplicitous as well, but I think this is terribly out of date."