Donald Tusk has said he is looking forward to discussing the EU’s cooperation “in detail” with Boris Johnson.
The outgoing president of the European Council congratulated the new prime minister on his appointment, and tweeted a letter to the new PM.
In the brief note, Mr Tusk said: “Dear Boris, congratulations on your appointment. I look forward to meeting you to discuss – in detail – our cooperation.”
Some commentators said Mr Tusk’s choice of words appeared to suggest he thought detail had been lacking with Mr Johnson.
Earlier, Brussels’ chief Brexit negotiator Michel Barnier said the European Union will be ready if Mr Johnson pursues a no-deal strategy.
He said he was waiting to hear from the Tory leader about his plans for Brexit, and stressed that a no-deal outcome would never be the EU’s preference.
Mr Barnier met MEPs on the European Parliament’s Brexit steering group (BSG) to discuss the incoming prime minister.
“It’s a very important moment for Brexit,” he told the BBC in Brussels.
“We look forward to hearing what the new prime minister Boris Johnson wants, what are the choices of the UK.
“Is it an orderly Brexit? This is the choice, the preference of the EU and we have worked for an orderly Brexit all along the last three years.
“Is it a no-deal Brexit? A no-deal Brexit will never be, never, the choice of the EU. But we are prepared.
“And for an orderly Brexit we will work along the next few weeks and months with the new UK Government in the best possible way in a very constructive spirit to facilitate the ratification of the Withdrawal Agreement.”
Mr Johnson has already declared that the Withdrawal Agreement is dead and that he will get the UK out of the EU – with or without a deal – on the October 31 deadline.
The BSG said statements made during the Tory leadership campaign had “greatly increased the risk of a disorderly exit of the UK”.
Following their meeting with Mr Barnier, the MEPs stressed their commitment to the Withdrawal Agreement – setting out the terms for the UK’s divorce from Brussels.
The BSG, chaired by Guy Verhofstadt, said “a no-deal exit would be economically very damaging, even if such damage would not be inflicted equally on both parties”.
A no-deal exit “will not be mitigated by any form of arrangements or mini deals between the EU and the UK”, they added.