David Cameron will attempt to persuade French president Francois Hollande of the pressing need for reform of the European Union over a pub lunch today.
The Prime Minister has promised to renegotiate the UK's relationship with Brussels ahead of a referendum on EU membership in 2017, but a source close to Mr Hollande signalled that it was "very, very unlikely" he would agree to treaty changes by that date.
The two leaders will discuss the issue at an informal lunch following a summit meeting at RAF Brize Norton in Mr Cameron's Oxfordshire constituency which will see progress on a number of joint defence projects including work on a new generation of combat drones.
British officials sought to play down the differences between the two men over the timing of EU reform, arguing that there was a growing consensus that changes were needed and the difference was over timing.
"We have seen over a period of months now increasing recognition around Europe about the need for treaty change," a source said.
"Every country will approach that from their own perspective in the usual way."
The source insisted there was an "entente tres cordiale" with France but "the Prime Minister will be making the points that he does around the importance of a more flexible, more competitive European Union".
"The Prime Minister's timetable is through to 2017," the source said.
"There is a debate that is ongoing, and I would simply point out that last year we were taking quite a few questions around whether or not there was a degree of consensus around the need for treaty change.
"I think you are seeing in a number of European capitals, there were discussions around this in the Bundestag recently as well, about the fact that the changes that the eurozone need to put through will require a treaty change.
"Each country will bring to the table its own perspectives and we will bring ours."
Mr Cameron was "optimistic and takes a very positive approach to the changes that he wants to see", the source added.
"If you take the arguments we are making about flexibility, and the importance of competitiveness, I think those are arguments that resonate across the EU.
"In terms of the timetable that we are working to that's through to the referendum in 2017.
"There is a process, we will continue to make our points over time."
The British and French governments will announce deals on nuclear power, space co-operation and defence projects at the summit.
The governments will announce a two year, £120 million feasibility study for a new armed drone, the Future Air Combat System.
They will also commit to a £500 million joint purchase of anti-ship missiles produced by MBDA and Finmeccanica.
The UK will also receive two A400M transport planes earlier than expected after swapping a delivery slot for the aircraft with the French.
The two countries will discuss the latest developments in the joint expeditionary force, which will see closer military co-operation between the British and French.
Joint exercises are planned later this year, a source said.
French energy giant EDF committed last year to build a new power station at Hinkley Point in Somerset, and the governments will sign a memorandum of understanding on involving small and medium-sized firms in the nuclear supply chain and co-operation between researchers on both sides of the Channel.
A framework agreement between the UK Space Agency and its French counterpart will include joint working on satellites and instruments.
The build-up to the summit has been dominated by revelations about the French president's private life, but a potential diplomatic banana skin has been avoided after it was confirmed there will be no official roles for political wives and partners.
M r Hollande ended a seven-year relationship with former first lady Valerie Trierweiler after a media report - which he has not denied - that he was having an affair with actress Julie Gayet.
Ms Trierweiler told Paris-Match, where she worked for a long time as a journalist, that she did not believe rumours about the affair until a gossip magazine report earlier this month.
"When I found out, it was like I had fallen from a skyscraper," she said.