Britain and France will sign an agreement for joint development and testing of nuclear weapons, the BBC reported on Tuesday.
According to the agreement, one centre will be set up in Britain to develop technology and another in France to carry out testing.
British Prime Minister David Cameron and French President Nicolas Sarkozy will also outline plans, at a London summit, for a joint army expeditionary force, the report 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," a Downing Street spokesperson was quoted as saying.
The summit comes two weeks after the British 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.
It is understood that each country will still control its own warheads, and that nuclear secrets will not be shared.
Both the countries have also agreed to keep at least one aircraft carrier at sea between them at any time. Each will be able to use the other's carrier in some form, certainly for training and possibly operations, the BBC said.
Meanwhile, France is to use British A400M fuelling aircraft when there is spare capacity, with plans in place for common maintenance and training.
Joint work on drones, mine counter-measures and satellite communications is also proposed.
In a statement, the French presidency said a 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.
Scientists from both countries would be able to ensure the "viability, safety and security in the long term of our nuclear arsenals", the statement said.
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."
Britain's shadow defence secretary Jim Murphy was quoted as saying: "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."
Murphy also questioned whether Britain was entering "an era where we are reliant on our allies to fill in the gaps in the government's defence policy".