"Please do not waste this time," Tusk said at a media conference after an eight-hour meeting with EU leaders.
"This extension is as flexible as I expected and a little bit shorter than I expected, but it's still enough to find the best possible solution," he said.
EU leaders and the U.K. government have agreed to a "flexible extension" of the Brexit deadline until Oct. 31. Tusk had earlier suggested giving Britain another 12 months to approve and ratify a deal on how the country would leave the EU.
From now until Oct. 31, the U.K. also has "the possibility to revoke article 50 and cancel Brexit altogether," said Tusk. He explained that Britain could leave the bloc earlier if members of its parliament find a consensus on a deal. The U.K. could also amend what it wants from a future trade pact with the EU, he added.
EU leaders would assess the situation surrounding Brexit again when they meet in June.
If British lawmakers don't ratify a deal by May 22, the U.K. must participate in European Parliament elections scheduled for May 23-26, according to the extension document. Failure to do so means the U.K. will leave the bloc on June 1, the document said.
Prime Minister Theresa May later told reporters that "nothing is more pressing or vital" than finding a deal agreed by U.K. members of Parliament. She added that she wants to take the country out of the EU "as soon as possible."