"'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 Tuesday on the outcome of Thursday's referendum at which Britons voted to leave the EU, prompting him to announce he will resign.
On Friday, 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 Saturday. 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 honour his pledge to launch Article 50 negotiations and respect the popular vote.
MUST BE EXPLICIT
The Council spokesman made clear that leaders cannot simply choose to interpret something Cameron says as the trigger without the prime minister saying clearly he means it to be.
"The notification of Article 50 is a formal act and has to be done by the British government to the European Council," the spokesman said. "It has to be done in an unequivocal manner with the explicit intent to trigger Article 50.
"Negotiations of leaving and the future relationship can only begin after such a formal notification. If it is indeed the intention of the British government to leave the EU, it is therefore in its interest to notify as soon as possible."
Since the shock vote on Thursday, won 52-48 percent by the Leave camp in defiance of polls and the bulk of the British establishment, there have been calls in Britain for the result to be reviewed or for parliament to ignore the referendum.
The second EU official, asked whether Britain could launch the process and then ask to stay, said that was not foreseen by the treaty: "Once you trigger it, you cannot take it back."
If a state fails to agree a departure treaty with the others, EU law simply stops applying to it after two years.
(Reporting by Alastair Macdonald; Editing by Bill Rigby)
By Alastair Macdonald