'No need to write, David,' impatient EU tells Cameron

"'Triggering' ... could either be a letter to the president of the European Council or an official statement at a meeting of the European Council duly noted in the official records of the meeting," a spokesman for the council of EU leaders said.

A second EU official, asked about mounting frustration among leaders with the British prime minister's delay in delivering the formal notification required to launch divorce proceedings, said: "It doesn't have to be written. He can just say it."

Cameron will brief the other 27 national leaders over dinner at a European Council summit in Brussels on June 28 on the outcome of June 24's referendum at which Britons voted to leave the EU, prompting him to announce he will resign.

On June 24, he said he would leave it to his successor as Conservative party leader and premier to trigger Article 50 of the EU treaty, which sets out a two-year process to quit the bloc. That appeared to be a reversal of a pledge to launch the process immediately after the vote. It has angered EU leaders who want a quick settlement to limit uncertainty.

Some European leaders still expect Cameron himself to start the process in the coming days or weeks, officials said on June 29. British officials were not immediately available.

Some Brexit campaigners have long said that Britain should aim to negotiate a comprehensive new relationship with the EU, seeking access to markets without submitting to EU rules or open migration, before binding itself into the two-year timetable that would be fixed for talks if Article 50 is triggered.

Such talk worries EU officials and leaders who fear that a prolonged haggling with London will further increase the risk of a domino effect of nationalist-led demands for exit from other states. They do not see a legal way to force Britain to start the process but have piled political pressure on Cameron to honor his pledge to launch Article 50 negotiations and respect the popular vote.