LONDON - French President Emmanuel Macron will hold his first summit with British Prime Minister Theresa May on Thursday (Jan 18) in London.

While relations between the two neighbouring countries remain close, new tensions are evident in the wake of Britain's decision to leave the European Union.

Military and security matters have always featured prominently, largely because this suited both sides. The French reserve their most important political and economic discussions for Germany, their closest ally in Europe. The British have tried for well over half a century to muscle in on this exclusive Franco-German link, but since they have invariably failed, they conduct a security dialogue with the French.

Britain and France do enjoy unique advantages - both account for Europe's largest standing armed forces, possess the continent's only nuclear arsenals and occupy permanent seats on the UN Security Council.

Back in 2010, the two countries concluded a unique 50-year military cooperation treaty; some of the provisions remain secret, particularly those which relate to the joint development and testing of nuclear weapons.

With defence budgets stretched in both countries - President Macron lost an army chief who resigned in protest against spending cuts, while Mrs May is under constant domestic criticism for not allocating more funds to her military - the French and British leaders have a vested interest in continuing to expand their defence cooperation.

And their intelligence agencies, facing the same threat of terrorism, have never worked in closer fashion. "Our partnership is one that we both rely on," remarked Mrs May on the eve of the summit.

Still, even in the security field, tensions are creeping in. Mr Macron has not hidden his irritation at what he sees as British indifference to French stability efforts in Africa. A large contingent of French troops have been deployed to Mali to prevent the African state's collapse, and Mr Macron's appeal to Europe to shoulder the burden has largely fallen on deaf ears in London.

The French also want the British to commit to a newly-created EU force which could be deployed quickly to zones of conflict. But the British are hesitant, partly because they feel that could overshadow Nato, the US-led military alliance in Europe, and partly because Mrs May would face a backlash from opponents of the EU inside her government, who hope that once Britain leaves the EU, it will not saddle itself with any new European obligations.

There are also wider areas of disaccord. The French president wants Britain to accept more asylum seekers from the Third World who are stranded in France on their way to the British Isles. On Tuesday, Mr Macron ostentatiously visited a migrant camp in Calais, the French port city facing the channel which separates the two nations.

The French leader came to power last year vowing to tear up a 2003 deal under which the British authorities gained the right to conduct immigration checks on French soil in order to prevent the entry of illegal immigrants.

Since then, Mr Macron has accepted that the treaty could continue, but is keen to disperse the refugee holding camps on the French side of the border.

"Calais is a land of passage, which has become a dead end for thousands of women and men who have spent years on the road," he said during his tour of the city.

Mrs May, who initially resisted even putting immigration on the agenda for the summit, may be able to offer additional cash on top of the €80 million (S$130 million) which the British already pay yearly for handling asylum seekers in France. But she is in no position to accept large numbers of migrants.

Beyond that, both leaders know that their biggest political haggling in decades is only just about to unfold, over the future relationship which Britain will have with the EU.

Mrs May hopes to leverage her country's continued military importance and the fact that trade between the two neighbours now stands at around S$132 billion a year against French support for a better trade deal to govern relations between the EU and Britain after Brexit next year.

But Mr Macron is in no hurry to offer concessions, partly because it suits France to continue exercising influence from within the EU rather than outside it. The French also calculate that Britain now needs them more than they need the British.

Either way, both Mr Macron and Mrs May are in no mood for the pomp and ceremonial which traditionally accompanies their summits. Their meeting will take place not in the gilded salons of London's royal palaces, but in the spartan and more secluded rural grounds of the Sandhurst Military Academy.