The EU's leader 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 favorite 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 said after a summit of EU leaders in Brussels.

He 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.