Britain's prime minister has warned that leaving the European Union would raise the risk of war, arguing that the U.K. would pay a high cost if it turns its back on the 28-nation bloc.

Cameron's major speech Monday on national security comes as the campaign ahead of the June 23 vote on the country's EU membership moves into its final weeks.

Cameron said that history shows that Britain cannot stand aside when Europe descends into conflict. He says "isolationism has never served this country well."

Arguing that peace itself could be jeopardized, Cameron said that 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.