The idea of an in-out referendum on European Union membership is gaining traction across the continent, as the debate on a possible British exit, or Brexit, intensified over the weekend.

British prime minister David Cameron is expected to say in a speech on Monday (9 May) that the chances of war in Europe would increase if British voters decided to leave the EU on 23 June.

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According to several of Monday's UK newspapers, the PM will invoke World War II.

“Isolationism has never served this country well,” Cameron will say.

“Whenever we turn our back on Europe, sooner or later we come to regret it. We have always had to go back in, and always at much higher cost.”

'Lose-lose' situation

The “extraordinary escalation” of the referendum campaign, as the Daily Mail put it, comes 45 days ahead of the vote, which appears to have inspired people in other EU states.

Some 45 percent of respondents to an Ipsos Mori poll carried out in eight EU countries said they wanted in-out referendums. Of the 6,000 people polled, 33 percent said they would vote to leave.

“The Italians in particular hope to have their own opportunity to go to the polls on their EU membership,” said Ipsos Mori researcher Bobby Duffy.

According to the poll, 58 percent of Italians and 55 percent of French wanted their own referendum, with between 41 and 48 percent of respondents saying they would vote to leave.

Many fewer respondents in Poland, Spain and Germany said they would support leaving the bloc.

A slight majority of respondents - 53 percent - thought that Britain would vote to stay in the EU.

European Commission president Jean-Claude Juncker and his economic and finance commissioner Pierre Moscovici share their belief.

“I wish passionately for Great Britain to remain in the European Union,” Moscovici told BFM TV.

“I think it would be lose-lose, a loss for them and a loss for us, if Britain left.”

“All Europeans want Britain to remain in the family,” Juncker said in an interview with the Funke Mediengruppe group, due out on Monday.

He said a Brexit would “surely have unforeseeable consequences on European cooperation, about which I absolutely do not wish to speculate because I am convinced that Britons will make the reasonable decision”.

Other political figures have tried to predict some of those consequences.

UK finance minister George Osborne said there would be a “significant hit” to the value of homes and to the cost of mortgages. He said the specific impact was still being investigated.

Meanwhile, two former top spies said the same day that Britain leaving the EU would make the country more vulnerable to terrorist attacks.

“Counterterrorism is a team game, and the EU is the best framework available - no country can succeed on its own,” wrote Jonathan Evans and John Sawers, two former chiefs of MI5 and MI6 in an article in the Sunday Times.

They added that Brexit would lead to “instability on the continent”.