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WATCH NOW: David Cameron and Francois Hollande news conference
Prime Minister David Cameron is expected to press French President Francois Hollande to back reform to the European Union at a UK-France summit.
The two men are holding talks on a range of issues, also including defence, energy and science, at the Brize Norton RAF base in Oxfordshire.
Sources said Mr Cameron would urge a "more flexible" EU during the meeting.
But a source close to Mr Hollande said he was unlikely to back changes to EU treaties to bring this about by 2017.
Mr Cameron's Conservative Party is promising to hold an in-out referendum on the European Union by the end of 2017 if it wins the 2015 general election.
Before this takes place, the Conservatives want to re-negotiate the UK's membership, returning more powers to the UK in areas such as immigration, welfare and justice.
'Optimistic'"Start Quote
End QuoteOf late the relationship has been pretty dire"
This is unpopular among Mr Hollande's ministers, according to BBC Europe editor Gavin Hewitt, who said: "They will not accept a Europe a la carte, where countries cherry-pick what they like."
UK officials sought to play down the differences between the two men, holding the first UK-France summit since Mr Hollande became president in 2012, saying: "We have seen over a period of months now increasing recognition around Europe about the need for treaty change. Every country will approach that from their own perspective in the usual way."
Mr Cameron was "optimistic and takes a very positive approach to the changes that he wants to see", a source added.
The prime minister is expected to make the argument for greater deregulation within the EU and a change of relationship between Brussels and countries not in the eurozone, including the UK.
However, a source close to Mr Hollande signalled that it was "very, very unlikely" he would agree to treaty changes by 2017.
After Friday's talks, the two leaders will hold a joint press conference and have a working lunch in a local pub.
The news conference will be Mr Hollande's first outside of France since he split with his partner Valerie Trierweiller amid reports he had an affair with another woman.
'Uncertainty'"Start Quote
End QuoteThe challenge for David Cameron is to frame his reforms in a way that might also appeal to the French voter and so perhaps weaken the resistance coming from the Elysee."
The summit follows criticism by both the UK and French governments over the state of the other's economies.
Some Conservatives have contrasted the stronger growth and falling unemployment in the UK with the sluggish recovery in France, blaming Mr Hollande for pushing up tax rates and failing to reform the public sector and suggesting Labour would take the same approach in the UK.
Labour's Chuka Umunna said this was counter-productive. "Tory politicians who have been gloating at France's economic challenges are not only undiplomatic but act against our national interest," he tweeted.
The two leaders are also expected to discuss the latest developments in the joint expeditionary force, which will see closer military co-operation between their respective armed forces.
Among a number of defence deals that were signed are a £120m feasibility study for a new armed drone, the Future Air Combat System, and a £500m joint purchase of anti-ship missiles
It was also announced that the UK will receive two A400M transport planes earlier than expected after swapping a delivery slot for the aircraft with the French.
A joint memorandum of understanding on nuclear power was also agreed, involving small and medium-sized firms in the nuclear supply chain and co-operation between researchers on both sides of the Channel.
Earlier this year, French firm EDF agreed to lead a consortium to build a new nuclear reactor at Hinkley Point in Somerset, the first new UK reactor in a generation.