The UK and France are to sign treaties agreeing to military co-operation including testing of nuclear warheads.
One centre will be set up in the UK to develop nuclear testing technology and another in France to carry it out.
Prime Minister David Cameron and President Nicolas Sarkozy will also outline plans, at a London summit, for a joint army expeditionary force.
No 10 called the measures “practical”, but Labour said they left “big questions” about the UK’s defences.
