Prime Minister Theresa May has asked for Brexit to be postponed to June 30 but Tusk said the evidence of recent months gave "little reason to believe" that divided British lawmakers would ratify the divorce deal by then, France24 reports.
"This is why I believe we should also discuss an alternative, longer extension," Tusk wrote, in a letter inviting EU leaders to the emergency Brexit summit he will host in Brussels on Wednesday.
"One possibility would be a flexible extension, which would last only as long as necessary and no longer than one year, as beyond that date we will need to decide unanimously on some key European projects."