Britain's shock vote to leave the EU is bad news for NATO and for Europe's defence cooperation in general, experts said ahead of this week's NATO summit.

"It's bad news for everyone," said Camille Grand, director of the Fondation pour la recherche strategique (RFS, or Strategic Research Foundation) in Paris.

"In NATO, the British will have less influence because the US will no longer necessarily rely on them," as Washington has long seen London as a useful lever to exert US influence within the EU, he added.

NATO Secretary General Jens Stoltenberg of Norway said he was "certain" that Britain would remain a key force in the alliance despite its June 23 Brexit vote to leave the 28-nation European Union.

One of his NATO predecessors, Dutchman Jaap de Hoop Scheffer, warned that the British vote "will negatively affect the close relationship between the EU and NATO which the UK had been a key part of."

Brexit also risks plunging Britain back into recession, threatening to cut its $60 billion defence budget, already hit by austerity cuts in recent years.

- Kremlin 'buoyed' by Brexit -

In the immediate term, the vacuum left by Brexit threatens above all to play into the hands of Russia, which sees any weakening of Europe as boosting its strategic interests.

"News of the referendum undoubtedly buoyed spirits in the Kremlin today, where leaders will view it as confirmation that Europe is weak and disunited," said Christopher Chivvis of the non-profit RAND Corporation.

He added: "This could encourage a more aggressive Russian stance against NATO in the future."

"But if Brexit is a near-term problem for NATO, it is a much bigger blow to the EU's long-struggling effort to build up its own security and defence capability," he added.

The EU's Common Security and Defence Policy (CSDP) will wither as a result of losing one of the continent's main military powers, experts said.

The United Kingdom and France are the only two European nations with substantial military assets including nuclear weapons and able and willing to intervene in external conflicts.

Britain heads up Europe's anti-piracy Operation Atalante off the Horn of Africa, helps hunt migrant smugglers in the Mediterranean and contributes to EU's Battlegroup rapid-reaction force.

- Europe divided -

"The most serious thing about Brexit, is that Europe will be divided and will therefore be unable to focus on strategic issues," said Grand.

Britain's negotiations to leave the EU will likely take years, even if the bloc's leaders would like it to go faster.

"It is likely that those who want to push Britain out the door as quickly as possible will clash with those who want to accommodate their wishes," said Pierre Razoux of the Institut de Recherche Strategique de l'Ecole militaire (IRSEM) in Paris.

The challenge is all the greater because Europe is facing the biggest threat since the end of the Cold War on its eastern flank, where Moscow is showing new appetites, and on its southern flank with growing pressure from migrants and jihadists.

The departure from the EU of Britain, fiercely opposed to military integration within the EU, could in theory boost European defence coordination long sought by EU heavyweights France and Germany.

- Major military power lost -

But creating a European military command and finding more money for EU operations, both blocked by London, will represent a "marginal" gain compared with the "loss of a major military power," said Grand.

Europe could find itself diminished at a time when the United States, more concerned by Chinese threats in Asia, is asking the Europeans to do more for their own defence.

To avert any such dangerous development, some argue that the EU and Britain should retain defence ties as close as possible.

"We have integrated Canadians and Georgians into European operations," said an aide to French Defence Minister Jean-Yves Le Drian.

"We should perhaps find some kind of closer association status so that the United Kingdom continues to commit itself to EU operations,