A shock exit by Britain from the EU without an agreement would be "a disaster", the European employers' organisation, Business Europe, warned on Monday.

One of the EU's most powerful lobbies, Business Europe issued the warning just hours before British Prime Minister Boris Johnson is scheduled to meet European Commission President Jean-Claude Juncker in Luxembourg.

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Both sides are under intense pressure to mend fences just a month and a half before the Britain's scheduled departure on October 31.

But Johnson, unlike his predecessor Theresa May, has said he is determined that the UK will leave the EU on October 31, with or without agreement.

"No deal is a recipe for disaster and should be definitely ruled out. A disorderly, no deal exit of the UK would be extremely harmful for all sides," said Markus J Beyrer, CEO of Business Europe in a statement.

Johnson on Friday said he was "cautiously optimistic" before his meeting with Juncker, the first with the EU executive chief since he came to power at the end of July.

A two-day European summit is scheduled to be held on October 17 in Brussels, which could be the last chance for an agreement between the two parties.

"We urge both sides to undertake all reasonable efforts and engage in constructive dialogue to avoid no-deal," Beyer said.

"In this context, we expect the UK government is able to bring viable options to the table as a matter of urgency," he added.