"We look forward to working constructively with PM Boris Johnson when he takes office, to facilitate the ratification of the Withdrawal Agreement and achieve an orderly Brexit," EU negotiator Michel Barnier tweeted after Johnson was confirmed as leader of Britain's Conservative Party and hence incoming premier.

"We are ready also to rework the agreed declaration on a new partnership in line with EUCO guidelines," Barnier said, referring to the non-binding political plan for future EU-UK ties that outgoing British leader Theresa May also signed in November.

Separately, a spokeswoman for the president of the European Commission Jean-Claude Juncker passed on his wishes.

"Mr Juncker ... wants me to convey my congratulations to Mr Boris Johnson on his appointment as leader of the Conservative and Unionist Party and as the next prime minister of the United Kingdom," she said.

"The president wants to work with the prime minister in the best way possible and beyond this, I'm going to reserve my comments as the news has just freshly broken."