Britain is in a "deep political crisis" following its referendum vote to leave the EU and so European leaders must give it some time to launch the formal divorce process, a senior EU official involved in preparing their next summit said on Sunday.

Briefing reporters ahead of Tuesday's meeting in Brussels, where Prime Minister David Cameron will speak to fellow leaders on the Brexit vote, the official said the EU did not expect him to make a formal notification of intention to withdraw from the bloc, and understood Britain needed time to organise itself.

"We, as the EU 27, stand ready to enter into this process swiftly and we expect the notification as soon as possible," the official said. "But ... everyone also understands that right now there is quite a significant crisis in the UK, a crisis which results from the outcome of the referendum.

"It's not only the crisis of the leadership of the ruling party ... but it goes much deeper."

Cameron plans to step down and wants to leave to his successor the task of triggering the two-year countdown to an EU exit by notifying the other states under Article 50 of the EU treaty. His Conservatives face a leadership battle, but so too does the Labour opposition, while Scotland's devolved government is trying to avoid being bounced out of Europe.

Asked about vocal pressure in Brussels, notably from EU lawmakers, for Cameron to start the withdrawal process right away, the official said fellow government leaders had more understanding for the position Cameron finds himself in.

"This is a very significant political crisis in the UK and expecting that this Tuesday there will be a kickstarting of the formal process under Article 50 is not really a realistic option," the official said of Tuesday's dinner in Brussels.

The following morning, on Wednesday, the other 27 will meet for the first time without Cameron to discuss how to proceed.

The official stressed that there would be no negotiation with Britain on the form its departure or a future trading relationship might take until Article 50 was triggered and repeated that EU leaders thought it was in the interests of both sides to do so swiftly to limit uncertainty.

"No notification, no negotiation," the person said.-Reuters