Britain could cooperate with EU defence operations even after it leaves the European bloc, Prime Minister David Cameron said Saturday.

"Outside the European Union, obviously we'll want to make sure that we play our full part in NATO operations," he told journalists in Warsaw, on the margins of a summit by the military alliance.

"And I'm sure that, you know, when there are EU operations that we agree with, there'll be opportunities perhaps for us to partner in some way," he added.

British voters shocked the European Union last month by choosing in a referendum to leave the 28-country bloc. Once Britain triggers Article 50 of the EU's treaty, which launches a divorce process of at least two years, the two sides will have to negotiate new arrangements to govern their future relationship.

Britain's departure from the EU, which has come to be known as Brexit, cast a shadow over the Warsaw summit.

Cameron said he assured NATO allies that his country would not be "turning our back on European security."

"While Britain may be leaving the EU, we're not withdrawing from the world," he added. "We'll continue to be an outward-looking nation that stands up for our values around the world."

The country is the second-largest military force in NATO after the United States and provides close to one-fourth of the total defence spending among European allies.

French President Francois Hollande on Saturday called for EU countries to increase their military spending.

"We have to contribute more to defence," he told journalists in Warsaw. "This is about providing the necessary resources so that we can all be stronger."

Cameron's comments came after survey results released on Friday showed that a majority of Britons are against holding a second referendum on Britain's membership in the bloc.

Of two thousand surveyed Britons, 40 per cent were in favour of repeating the June referendum, while 44 per cent were against, according to the survey conducted by ORB International pollsters on behalf of the news website The Independent.

Over 4 million Britons had signed an online petition calling for a repeat of the in-out referendum, with tens of thousands marching in London to protest against a British exit, or Brexit, from the EU.

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