The Europian Union's leaders on Friday fired a Brexit warning to whoever wins the battle to become the next British prime minister, insisting the existing divorce deal will not be changed.

Hot favourite Boris Johnson faces foreign minister Jeremy Hunt in a run-off vote to decide who takes on the tricky task of piloting the country's departure from the EU.

Both say they want to renegotiate the deal that outgoing British Prime Minister Theresa May struck with Brussels after two years of painful negotiation — a deal which British lawmakers have rejected three times.

At a leaders' summit in Brussels, European Council President Donald Tusk said the bloc would remain “very precise and also patient” despite the high political drama unfolding in Westminster.

“Maybe the process of Brexit will be even more exciting than before because of some personnel decisions in London, but nothing has changed when it comes to our position,” Tusk told reporters.

He said that all remaining 27 EU leaders were adamant there could be no changes to the legal accord struck in November last year. “We are open for talks when it comes to the declaration on the future UK-EU relations if the position of the UK were to evolve, but the withdrawal agreement is not open for renegotiation,” Tusk said.

Jean-Claude Juncker, the head of the European Commission, which led Brexit talks for the EU side, said leaders “repeated unanimously there will be no renegotiation of the withdrawal agreement”.

Divisive Johnson

Barring a major upset, the 160,000 grassroots members of the Conservative party will choose Johnson to take over from May in July. But the blond former mayor of London is a hugely divisive figure.

Some say his wit and charisma is what is needed to win Brussels around, while critics point to his long history of gaffes and accuse him of cheap populism and a lazy lack of attention to detail.

Recent threats by Johnson to withhold Britain's 39-billion-pound (44-million-euro, $50-million) divorce bill unless the EU agrees to better terms were seen in Brussels as a sign of possible bad faith.

Though she was berated at home for her plodding approach, May won admirers in the EU for what they saw as her fair-minded pursuit of a Brexit deal.

French President Emmanuel Macron, who has taken a tough line on Brexit, said he hoped May's successor would act with the same “formidable loyalty and formidable respect” she had shown.

“She never sought to block Europe and she never held discussions about the future hostage,” Macron said.

“I don't want to involve myself in internal British life (but) I hope we will see the same decency and the same spirit of responsiblity which she brought.”

Brexit has been delayed twice already and both Hunt and Johnson say Britain should leave the bloc on the current deadline date of Oct 31 — even if it means walking away with no deal. But Hunt has suggested he might delay Brexit briefly if a deal with Brussels was close, and Johnson has refused to absolutely guarantee leaving on Oct 31.

Border problem

Patience across the channel is wearing thin, with Irish Prime Minister Leo Varadkar warning at the Brussels summit on Thursday of “enormous hostility” in the EU to granting Britain yet another delay.

One of the main sticking points has been how to prevent a “hard border” with customs checks on the border between EU member Ireland and British-ruled Northern Ireland.

The withdrawal agreement includes a “backstop” that keeps Britain in the bloc's customs union until a better solution is found, but hardline anti-EU MPs see this as a trap to stop the UK escaping.

Some have suggested “alternative arrangements” could be found, but — not for the first time — Varadkar noted that so far no-one had come up with concrete proposals or proved they would work.

“That has not been done yet and I don't see that being done this side of October 31,” Varadkar told reporters, insisting the backstop must stay.