BRUSSELS: The EU's top leaders Friday fired a Brexit warning to whoever wins the battle to become the next British prime minister, saying the existing divorce deal could not be changed.

Hot favourite Boris Johnson will face 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 PM Theresa May struck with Brussels after two years of painful negotiation, a deal which British lawmakers have rejected three times.

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 personal decisions in London but nothing has changed when it comes to our position," Tusk told reporters after a summit of EU leaders in Brussels.

The former Polish premier said that all remaining 27 EU leaders were adamant there could be no changes to the near 600-page legal accord struck in November last year.

"We look forward to working together with the next UK prime minister," Tusk said.

"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."

Jean-Claude Juncker, the head of the European Commission, which led Brexit talks for the EU side, said there was nothing to add because leaders "repeated unanimously there will be no renegotiation of the withdrawal agreement".

Brexit has been delayed twice already and both Hunt and Johnson say Britain should leave the bloc on the current deadline date of October 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 October 31.

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. —AFP