The UK and France are to sign treaties agreeing to military cooperation including testing of nuclear warheads.
The plans will see one centre set up in the UK to develop technology and another in France to carry out testing.
Prime Minister David Cameron and President Nicolas Sarkozy will also outline plans, at a London summit, for a joint army expeditionary force.
Downing Street called the measures "practical", but Labour said they left "big questions" over the UK's defences.
A Downing Street spokesman said: "This summit marks a deepening of the UK-France bilateral relationship. Ours is now a strategic partnership tackling together the biggest challenges facing our two countries."
The summit comes two weeks after the UK government announced cuts to its armed forces, in the first strategic defence review since 1998, as part of savings aimed at reducing the country's budget deficit.
Under the plans £750m will be saved over four years on the Trident nuclear missile system by cutting the number of warheads.
Harrier jump jets, the Navy's flagship HMS Ark Royal and planned Nimrod spy planes will also be axed, but two new aircraft carriers were spared.
Mr Cameron and Mr Sarkozy are to sign two treaties - one on greater general military co-operation and the other on nuclear weapons.
Speaking about greater military co-operation, Defence Secretary Liam Fox said there had been a "great deal of hysteria" in the media about the idea of British troops coming under French command.
He said: "Under the existing Nato system our troops could come under Turkish or Polish command. There's nothing new about that.
"This does not affect our special relationship with the United States. It gives us economies of scale and helps us to welcome France back fully into Nato."
Asked whether there might be a situation in the future, during a political crisis in which Britain and France did "not see eye to eye", when British planes were dependent on a French aircraft carrier, he said: "That's very unlikely to happen."
"This is not about interdependence. It's about interoperability," he added.
Operational veto
The nuclear treaty will establish a centre in the UK to develop technology and another one in France to carry out the testing.
It is understood that each country will still control its own warheads, and that nuclear secrets will not be shared.
The other treaty will allow the setting up of a "combined joint expeditionary force", thought to involve a brigade of about 5,000 soldiers from each side.
Each country will retain a veto for each operation, which will operate under one military commander to be chosen at the time.
The UK and France have also agreed to keep at least one aircraft carrier at sea between them at any one time.
Each will be able to use the other's carrier in some form, certainly for training and possibly operations.
Meanwhile, France is to use British A400M fuelling aircraft when there is spare capacity, with plans in place for common maintenance and training.
'Deepening ties'
Joint work on drones, mine counter-measures and satellite communications is also proposed.
In a statement, the French presidency said the test centre in Valduc, eastern France, would start operations in 2014.
The Valduc laboratory would work with a French-British research centre based in Aldermaston, Berkshire, it added.
Together the facilities would involve "several dozen" French and British experts and cost both countries several million euros.
It said scientists from both countries would be able to ensure the "viability, safety and security in the long term of our nuclear arsenals".
Mr Cameron told MPs on Monday: "I do seriously believe that this link-up with the French over defence is in the long term interests of both our countries.
"And to those who worry that this might in some way lead to... European armies, that is not the point. The point is to enhance sovereign capability by two like-minded countries being able to work together."
The UK's shadow defence secretary Jim Murphy said: "I support the government's emphasis on international co-operation, taking forward the good work of the last government.
"We share common threats with countries such as France, from terrorism to privacy to cyber-attack. Deepening military ties is an essential part of modern defence policy.
"Interdependence, however, is different from dependence, and binding legal treaties pose some big questions for the government."
Mr Murphy also questioned whether the the UK was entering "an era where we are reliant on our allies to fill in the gaps in the government's defence policy".
Michael Clarke, director of the Royal United Services Institute think tank, said Anglo-French military initiatives dated back to Dunkirk and he said: "Both (countries) have got similar problems. They have both got similar interests... and the fact is that if both powers are determined to play a role in the world with military force they are going to find ways of pooling their relatively common force structures if they are going to have some effect."
Asked about hypothetical scenarios where France and Britain disagreed politically, Professor Clarke said: "You can't rule them out but they are not the most likely outcome. Most things that happen in the world at the moment - in Afghanistan, in southern Europe, in relation to instabilities in eastern Europe - are things that the British and French do share common interests on."
