The EU says Britain has the right to trigger Article 50, which sets a two-year deadline for a deal, by making a formal declaration either in a letter or a speech.
It is to be noted that UK PM David Cameron has said he will resign by October to allow his successor to conduct the talks, but Britain has been urged by the EU foreign ministers to start the process soon.
A spokesman for the European Council, which defines the EU's political direction and priorities, said that triggering Article 50 was a formal act which must be "done by the British government to the European Council".
"It has to be done in an unequivocal manner with the explicit intent to trigger Article 50," the spokesman said.
"It 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."
On Saturday, German Chancellor Angela Merkel said the EU had "no need to be particularly nasty in any way" in the negotiations with Britain.
"It shouldn't take forever, that's right, but I would not fight for a short timeframe," she said.