David Cameron sounded a warning ahead of the EU referendum | Chris Ratcliffe/Getty Images David Cameron: Brexit could lead to war Peace can’t be guaranteed among countries that have been ‘at each others throats for decades.’

A British exit from the EU could put the Continent at risk of war, Prime Minister David Cameron is expected to say in a speech in London Monday.

Cameron will argue the EU has helped unite countries that had been "at each others throats for decades," according to British papers, which previewed parts of the speech Monday morning.

"Can we be so sure that peace and stability on our Continent are assured beyond any shadow of doubt? Is that a risk worth taking? I would never be so rash as to make that assumption," Cameron is expected to say.

"The truth is this: what happens in our neighborhood matters to Britain."

"That was true in 1914, in 1940 and in 1989. Or, you could add 1588, 1704 and 1815.... And if things go wrong in Europe, let's not pretend we can be immune from the consequences."

Cameron's address will be followed by Leave campaigner and former London Mayor Boris Johnson, who will outline the “cosmopolitan case for Brexit,” the BBC reported.

Cameron has at least one high-ranking supporter: European Commission President Jean-Claude Juncker echoed the British prime minister's warning in an interview with German media, published Monday.

Juncker said a Brexit after the June 23 referendum would "surely have unforeseeable consequences on European cooperation." He added: "I absolutely do not wish to speculate about [those consequences] because I am convinced that Britons will make the reasonable decision.”

A new Ipsos MORI poll found 58 percent of Italians and 55 percent of French already wanted a referendum on their own EU membership, regardless of the outcome of the British vote, the FT reported.