The bloc's executive European Commission was willing to work with Johnson, a spokeswoman said, but the limits were clear.

"We look forward to working constructively with PM Johnson when he takes office, to facilitate the ratification of the withdrawal agreement and achieve an orderly Brexit," said the bloc's negotiator of the unprecedented divorce, Michel Barnier.

"We are ready also to rework the agreed declaration on a new partnership," he added, referring to a political declaration that accompanies the legal withdrawal agreement.

Minutes before Johnson's victory was announced in the Conservative Party leadership contest, the Commission's deputy head Frans Timmermans said the EU would not agree to change the deal it had sealed with outgoing British leader Theresa May. That deal was rejected three times by the British parliament.

"The United Kingdom reached an agreement with the European Union and the European Union will stick to that agreement," Timmermans told a news conference. "This is the best deal possible."

Timmermans said the EU would hold the line on Brexit and that Johnson's flamboyant "character or persona or attitude" made no difference.

The EU is bracing for a no-deal Brexit, or another delay to Britain's departure, should Johnson follow through on his promises as Britain's leader.

"We will hear what the new prime minister has to say when he comes to Brussels," Timmermans said, but warned against the most-damaging scenario of the United Kingdom leaving without an agreement in place to manage the fallout.

Johnson has promised to deliver Brexit on October 31, with or without a deal.

"A no-deal Brexit, a hard Brexit, would be a tragedy - for all sides, not just for the United Kingdom," Timmermans said.

"We are all going to suffer if that happens."

Australian Associated Press