French President Emmanuel Macron on Friday warned London that agreement on the crucial issue of Britain's EU exit bill was still "a long way off".

As EU leaders gave beleaguered British Prime Minister Theresa May hope by agreeing to start internal work on the bloc's relationship with Britain after Brexit, Macron injected a dose of scepticism into the optimsitic mood.

"We are a long way off on the financial commitments," he said after the summit in Brussels.

"There is major work to be done on the United Kingdom's side," to settle the divorce bill, which has been the major sticking point in five rounds of talks between London and Brussels.

"Today we are not even halfway down the road."

The 27 EU leaders -- minus May -- agreed on Friday to begin internal preparations for trade talks even though not enough progress has been made on the terms of the divorce.

On the finance issue, European capitals are demanding detailed written commitments on finance before progressing to trade talks, fearing that Brexit will blow a hole in the bloc's budget.

May has said Britain will maintain its contributions for two years after Brexit to complete the current EU budget period, totalling around 20 billion euros ($24 billion).

But Macron said this was not enough and London must also take into account commitments it made to fund future EU projects.