BRUSSELS — European Union lawmakers pressed Britain to end the uncertainty gripping European and global markets, saying today that if it intends to leave, it should start the process immediately.

Prime Minister David Cameron entered talks with European Commission President Jean-Claude Juncker hours ahead of an EU summit which is expected to hear from the British leaders that exit talks might not be launched before October.

Juncker and other European leaders insisted they won’t begin any talks until Britain invokes the EU Constitution’s Article 50, which sets in motion a two-year process to split.

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. “We cannot allow ourselves to remain in a prolonged period of uncertainty.”

French far-right leader Marine Le Pen called the British vote “an extraordinary victory for democracy — a slap for a European system based more and more on fear, blackmail and lies.”

German Chancellor Angela Merkel said she will use “all her strength” to prevent the EU from drifting apart. She and other EU leaders were beginning a two-day summit today to chart the way forward.