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Raising the stakes in Britain's European Union membership debate, Prime Minister David Cameron said on Monday that leaving the bloc would increase the risk of war in Europe.

Cameron's speech on national security came as campaigning ahead of a June 23 vote on the country's EU membership moved into its final weeks.

Cameron said it would be rash to assume that "peace and stability on our continent are assured beyond any shadow of doubt."

He said the EU "has helped reconcile countries which were at each other's throats for decades."

"Britain has a fundamental national interest in maintaining common purpose in Europe to avoid future conflict between European countries," he said.

Cameron argued that "isolationism has never served this country well. Whenever we turn our back on Europe, sooner or later we come to regret it."

The EU was founded after World War II, in part to prevent the continent's nations from waging war ever again.

But attacks in Paris and Brussels and the arrival of hundreds of thousands of refugees from the Middle East, Asia and Africa have highlighted the security challenges facing the bloc.

The in Britain's referendum debate have argued bitterly over whether leaving the 28-nation union would make Britain more or less secure.

Two former UK intelligence chiefs said in an interview published on Sunday that the EU gives Britain an edge in gathering anti-terror intelligence and underpins continental peace.

Cameron said that, with the fight against the Islamic State group raging and an assertive Russia flexing its muscles, "now is a time for strength in numbers."

The anti-EU group called Cameron's claims "historically illiterate," arguing that NATO contributes far more to European peace and security than the EU.

"The notion that the peace of Europe was guaranteed by the institutions of the European Union was always utter nonsense," said Robert Cowcroft of the University of Edinburgh, a member of Historians for Britain, an anti-EU group.

Former London Mayor Boris Johnson, a leader of the Vote Leave campaign, said the 1990s Balkan wars on the EU's doorstep showed the bloc was no guarantee of peace.

"I saw the disaster when the EU was charged with sorting out former Yugoslavia, and I saw how NATO sorted it out," Johnson said in a London speech.

"I don't believe that leaving the EU would cause World War III to break out on the European continent."