Britain and France have agreed to extend defence and nuclear co-operation as David Cameron held talks with President Francois Hollande at an RAF air base in the Prime Minister's Oxfordshire constituency.
Mr Cameron announced a two-year £120 million feasibility study for a new armed drone, the Future Air Combat System, and said the UK and France will work together on an unmanned counter-mine craft.
Britain will offer more logistical support for the French military mission in the war-torn Central African Republic, and troops will hold a joint training exercise later in the year. Meanwhile, the UK will try out the French VBCI tank with a view to possibly purchasing it for the Army.
Speaking alongside Mr Hollande at a press conference following their talks at RAF Brize Norton, Mr Cameron described the cross-Channel partnership as "as close and important as ever".
The Prime Minister said: "We recognise that if we, Britain and France, do more together, our defence budgets will go further, our armed forces will benefit from better equipment and our defence industries will remain world leaders and we will be able to have a greater global impact.
"Today we have agreed important progress on the landmark Lancaster House treaty. We will stay at the forefront of defence technology by investing £120 million together in the feasibility phase of an unmanned combat air vehicle.
"We will work together to design a new unmanned maritime vehicle to counter seabed mines. We will strengthen the ability of our armed forces to work together overseas by sharing experience on unmanned surveillance aircraft and by holding a land exercise for our joint expeditionary force later this year.
"The president has also agreed that we can test their latest armed vehicle - the VBCI, which we saw this morning - which if we choose to purchase would also increase the ability of our forces to fight alongside one another."
Referring to the French actions in Mali and CAR, Mr Cameron said: "Today I have offered further logistical flights and air-to-air refuelling support to support these vital French missions."
Turning to Syria, the PM said they wanted to address the risk of terrorism in Britain and France due to the conflict.
"We have agreed to work together to tackle the security risk posed by UK and French nationals who have travelled to Syria for jihadist fighting and then seek to return here," he added.
Mr Cameron said the leaders had a "shared interest in making Europe more competitive and driving economic growth".
"This is the number one priority for both of us and we are both taking difficult decisions to secure recovery for all," he said.
"I believe the policies which the president recently announced, to cut taxes on business, reduce employment costs and remove unnecessary regulation are the right way to boost investment and create jobs."
The Lancaster House Treaties were signed in 2010, and set out a framework for closer defence and security cooperation between the countries.