French president Francois Hollande has told David Cameron that Paris does not regard renegotiation of European Union treaties as a "priority".
In talks at RAF Brize Norton, the Prime Minister made clear that he believes changes to the fundamental EU treaties are necessary as part of the process leading to the in/out referendum on British membership he has promised by the end of 2017.
Differences over Europe overshadowed a UK-France summit which saw agreements to extend defence co-operation, including a £120 million joint feasibility study for a new armed drone and additional British logistical support for France's military mission in the Central African Republic.
Mr Hollande was also confronted with questions from the British press about his reported affair with actress Julie Gayet, but refused point-blank to discuss his private life.
Daily Telegraph senior political correspondent Christopher Hope asked Mr Hollande: "Do you think your private life has made France an international joke, are you still having an affair with Julie Gayet and do you wish she was here?"
The president replied with the terse comment: "In regard to your last question, I'm afraid I decline to answer."
Mr 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 Ms Gayet. Potential awkwardness over his relationships was avoided by the fact that spouses were not invited to the summit.
The French president was more expansive on the issue of European reform, insisting that there was no urgent need for treaty change.
"France would like the UK to remain within the EU," said Mr Hollande. "France would like to have a more efficient Europe which can attain the objectives which we consider to be essential - growth, employment, energy and, of course, the capacity to bring in the techniques for tomorrow and to protect our population.
"France would like the eurozone to be better co-ordinated, better integrated and, if there are going to be amendments to the text, we don't feel that for the time being they are urgent. We feel that revising the treaty is not a priority for the time being."
But Mr Cameron said that there had already been two changes to treaties since he took office in 2010 and insisted: " Just as the eurozone needs change, so Britain wants change in Europe - change for all of Europe to make Europe more competitive, more flexible, better able to succeed in this global race, but also changes that Britain wants to see.
"My position absolutely remains that we want to see those changes, we want to see that renegotiation. That renegotiation will involve elements of treaty change and then there will be a referendum in Britain before the end of 2017 that is an in/out referendum. People need to know that is absolutely what we will achieve and I believe it will be done and I believe it will be important for Europe and important for Britain.
"I want to see Britain vote to be a member of a reformed European Union."
Differences over economic policy also surfaced, with Mr Hollande indicating a preference for state intervention to boost growth as he said: "Now we are in the period of recovery, but we can't just wait for that recovery to happen, we have to stimulate it, to amplify it."
Conservatives have been critical of Mr Hollande's economic policies, including the introduction of a 75% top rate of tax, with party chairman Grant Shapps saying earlier this month that the president had run France "into the sand".
Mr Cameron said he "commended" Mr Hollande's efforts to cut spending and use the proceeds to reduce taxes and unemployment.
But he added: "Of course we are not going to agree about everything. Francois is a French Socialist, I am a British Conservative - it would be odd if we agreed about everything.
"But on this point we do agree - that we need structural reform and changes in our economy to improve growth, to improve employment and to give our people the best chance of having that security and stability of a regular pay packet so they can provide for their families. The ends we seek are the same."
The two leaders were due to discuss Europe over lunch in a Cotswold country pub following the summit in the RAF air base in Mr Cameron's Oxfordshire constituency.
Mr Cameron said that the cross-Channel partnership was "as close and as important as ever" as he set out details of new agreements on military co-operation, building on the Lancaster House agreement which he signed with Mr Hollande's predecessor Nicolas Sarkozy in 2010.
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 the CAR, Mr Cameron said: "Today I have offered further logistical flights and air-to-air refuelling support to support these vital French missions."
Labour Europe spokesman Gareth Thomas said: "Twelve months on from his Bloomberg speech, David Cameron's referendum policy is already fraying at the seams. His own backbenchers are baying for more, while other EU members have dismissed his arbitrary timetable of 2017.
"Europe does need to change, but the UK's partners are already ruling out treaty change on the arbitrary timetable that David Cameron has set out.
"David Cameron's approach to Europe risks leaving the UK more isolated in the EU and undermines Britain's ability to secure much-needed reform in Europe."