The European Union ratcheted up pressure Tuesday on the U.K. to trigger negotiations to leave the bloc and end the uncertainty that has rattled stock markets.

In Berlin, German Chancellor Angela Merkel said she will use "all her strength" to prevent the EU from drifting apart in the wake of Britain's decision.

At an emergency session in the European Parliament hours ahead of a summit of EU leaders, EU Commission President Jean-Claude Juncker called on Britain to clarify its future, after Prime Minister David Cameron suggested that exit talks might not be launched before October.

"I want the U.K. to clarify its position. Not today, not tomorrow at 9 a.m., but soon. We cannot allow ourselves to remain in a prolonged period of uncertainty," Juncker told EU lawmakers.

Juncker said that he had banned policy commissioners under his command from holding any secret talks with Britain on its future until London triggers the exit clause known as Article 50 that launches negotiations on Britain's departure.

"No notification, no negotiation," he said, hours before EU leaders begin a two-day summit in Brussels to hear Cameron's position and chart the way forward.

Once Article 50 is triggered, the U.K. would have two years to negotiate its exit, unless all remaining 27 EU nations agree to extend that period. The talks would take into account the future relations envisaged between the EU and Britain, but a new round of negotiations, potentially years long, would be required to finalize that new relationship between them.

In an address to the German Parliament before heading to Brussels, Merkel said she expected that Britain would want to maintain "close relations" with the EU once it leaves, but also warned that it could not expect a business as usual approach.

"Whoever wants to leave this family cannot expect to have no more obligations but to keep privileges," she said. Some in Britain are hoping that the nation could still enjoy all the perks of the seamless EU internal market for business, while being able to deny EU citizens entry to Britain at will.

"We will ensure that the negotiations are not carried out with the principle of cherry picking," Merkel said.

She joined Juncker in underlining that there can be no talks with Britain on leaving the EU until Britain starts formal procedure to leave.

While acknowledging the need for haste, the EU's Dutch presidency called for some patience, given the political chaos the exit referendum has caused in Britain, dividing both the governing and opposition parties and sending the pound plummeting.

"No one, no one, will benefit from a period of prolonged limbo. The ball is in London's court," Dutch Defense Minister Jeanine Hennis-Plasschaert told the EU lawmakers, but she warned: "Cool heads must now prevail."

Lawmakers paid tribute to Britain's commissioner in Brussels, Jonathon Hill, who resigned after last week's vote and wept in the parliament Tuesday as he received a standing ovation.

During the unprecedented session, lawmakers are set to call for Britain to trigger the exit process immediately.

A draft resolution drawn up by party leaders says the process should be launched as soon as Cameron notifies the outcome of the British referendum to EU leaders. The non-binding resolution could be modified before it is adopted.

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David Rising in Berlin contributed.