European Union chief Donald Tusk says talks on Britain's departure from the EU remain the bloc's toughest test and he is renewing calls for unity among the U.K.'s 27 partners.

European Council President Tusk said Tuesday that Brexit "is still the toughest stress test. If we fail it, the negotiations will end in our defeat."

Debriefing EU lawmakers in Strasbourg, France on last week's EU summit, Tusk says "it is, in fact, up to London how this will end: with a good deal, no deal or no Brexit."

He adds that "the EU will be able to rise to every scenario as long as we are not divided."

Britain is set to leave the EU in March 2019, but the Brexit talks are progressing very slowly.