"The main political problem with Brexit is the situation inside Britain ... the emotions related to this show that we are all victims of Britain's internal politics," Tusk told private broadcaster TVN24.
"It cannot be ruled out that Boris Johnson will want to lead to an exit at all cost, just because of internal interests," he added.
Johnson's Brexit plan was facing mounting legal, political and diplomatic challenges on Friday as Ireland accused Britain of being unreasonable and former British leader John Major sought to stop the suspension of parliament.The ultimate outcome of Britain's tortuous three-year Brexit crisis remains unclear with options ranging from a frantic departure without an exit deal or a last-minute agreement to an election or referendum that could cancel the whole endeavour.