European Commission chief Jean-Claude Juncker called Tuesday on Prime Minister David Cameron to clarify quickly when Britain intends to leave the EU, saying there can be no negotiation on future ties before London formally applies to exit.

"I will see the prime minister later this morning... to ask him to clarify the situation as rapidly as possible. We cannot get into a period of extended uncertainty," Juncker told the European Parliament, adding: "No notification, no negotiation".