President of the European Council Donald Tusk congratulated Boris Johnson on becoming British prime minister Wednesday but immediately stressed that the EU needed to hear "details" of his Brexit plan.

"On behalf of the European Council I would like to congratulate you on your appointment as Prime Minister of the United Kingdom of Great Britain and Northern Ireland," the former Polish leader wrote.

"I look forward to meeting you to discuss - in detail - our cooperation," he added, in a letter that was barely longer than the equally terse tweet he had just sent announcing it.

Tusk, whose Council represents the leaders of the 28 EU member states, has made no secret of his dismay at Britain's decision to quit the European Union and was instrumental in securing delays to Brexit day itself in the hope London would change its mind.

Instead, the Conservative Party chose arch-brexiteer Johnson as its leader and hence prime minister, and the new tenant of Number 10 insists that Britain will leave the EU on October 31 whether or not he has negotiated a new withdrawal agreement.

Tusk's insistence on "detail" recalls a previous outburst when he declared that there was a "special place in hell" for those - like Johnson - who campaigned urging British voters to back Brexit "without even a sketch of a plan of how to carry it out safely".

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