BRUSSELS — European Union leaders began plotting a future without Britain on Tuesday, urging the island nation and economic powerhouse to disentangle itself as fast as possible from the other 27 nations in the bloc to avoid extending the turmoil that has been roiling European and global markets.

EU Council President Donald Tusk said he was planning a special meeting of the EU leaders minus Cameron in Bratislava in September to chart a way ahead, after last week’s referendum made abundantly clear that a business-as-usual approach to Britain leaving could possibly threaten the unity of the entire bloc.

On Tuesday, Prime Minister David Cameron held talks with European Commission President Jean-Claude Juncker hours ahead of an EU summit in Brussels where the outgoing British leader is expected to say that exit talks might not be launched before October. There has been talk that Britain wants informal negotiations on what the U.K.’s future relations with Europe might look like before that happens — a notion many in the bloc have rejected.

Juncker and other European leaders insist they won’t begin any talks until Britain invokes the Article 50 of the EU’s Treaty of Lisbon, which sets in motion a two-year process to split away from the group designed to unify Europe after the horrors of World War II.

In an unprecedented emergency session of the EU parliament, called after Britain voted Thursday to leave the union, Juncker demanded that Britain clarify its future.

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

Juncker said he had banned his policy commissioners from holding any secret talks with Britain on its future until London triggers the exit clause.

“No notification. No negotiation,” he said to resounding applause.

Tusk was already looking further ahead. He said the 27 EU heads and state and government — minus Cameron — would hold a special meeting in September to discuss “the new process of deeper reflection, a new impulse for Europe, a new future for Europe.”

“We need a few weeks to prepare this process,” Tusk said.

The immediate reaction to the British departure — also called the Brexit — in the EU parliament was emotional Tuesday. Nigel Farage, a British member of the European Parliament and a leader in Britain’s “leave” movement, was booed and jeered when he urged Europe to give Britain a good trade deal when it leaves, saying jobs in Germany’s auto sector might be at stake if it doesn’t.

“Why don’t we just be pragmatic, sensible, grown-up, reasonable … and cut a sensible tariff-free deal?” he asked.