Boris Johnson's shock appointment as Britain's foreign secretary has given European counterparts an immediate diplomatic headache - over whether to have dinner with him on Sunday in Brussels.

EU diplomats said ministers had planned a quiet chat on post-Brexit relations with Johnson's predecessor, expecting Philip Hammond, who campaigned to stay in the EU in last month's referendum, to reassure them on continued cooperation in crises such as Libya.

But those supper plans, proposed by EU foreign policy chief Federica Mogherini to precede a routine ministerial council she will chair on Monday, have been thrown into disarray by the sudden acceleration in the British political process and the appointment of controversial Brexit campaigner Johnson.

Four EU and British sources told Reuters on Thursday it was unclear the dinner would go ahead and if so, who among Johnson and the 27 other EU ministers would attend.

"The dinner and its timing is starting to look awkward," one EU diplomat said. "It may yet be cancelled, although that is for Mogherini to decide." Mogherini's office declined to comment.

A former Brussels journalist and mayor of London, Johnson cultivates a clownish public image but also caused deep offence in the European Union during the referendum campaign by comparing its aims to unify Europe to those of Adolf Hitler.

Hours after May appointed him in an apparent effort to heal rifts in her Conservative party, France's foreign minister called Johnson a liar and the German foreign minister described Johnson's behaviour as "outrageous".

Among Brussels diplomats the response was more muted: "He's a surprise choice and there are questions about whether he is credible," one envoy said.

The others still expect to see him at Monday's council, he said, adding: "Ministers will try to keep an open mind."-Reuters