"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."
May has been holding talks with Jeremy Corbyn, leader of the opposition Labour Party, to find a compromise or alternative plan to Brexit. Mary Creagh, a British lawmaker from the Labour Party, said the extension to Oct. 31 gives the U.K. "a chance to take a breath" and an opportunity to press on with cross-party talks.
"I do think it gives Parliament and MPs (members of Parliament) and the government a chance to take a breath and to now begin and negotiate in good faith. But we need confidence-building measures, frankly, and we haven't seen a lot of that. We haven't seen that good faith from the prime minister so far in this process," she told CNBC's "Squawk Box."
The latest extension in the Brexit deadline is the second delay that the EU and the U.K. have agreed on. Britain was supposed to leave the EU on March 29, but the deadline was extended after British lawmakers failed to back the prime minister's withdrawal deal.
Under the first extension agreement between the U.K. and the EU, Britain would leave the bloc with May's deal on May 22 or without any deal on April 12.
Many EU leaders had wanted a much longer extension to the end of this year, or even March 2020. But French President Emmanuel Macron said he took responsibility for blocking such a long delay to the Brexit process.
"It's true that the majority was more in favor of a very long extension. But it was not logical in my view, and above all, it was neither good for us, nor for the U.K.," he said. "I take responsibility for this position, I think it's for the collective good."
German Chancellor Angela Merkel said the latest extension to Oct. 31 allows the U.K. the best chance of an orderly exit from the EU. European Commission President Jean-Claude Juncker also said he would like to see the Britain leave with a deal, or don't leave at all.
Dutch Prime Minister Mark Rutte said he expects the latest extension to be the last one. "On October 31, the British will either have agreed to a deal, have decided to cancel Brexit or leave without a deal," he said.
— Reuters and CNBC's Eustance Huang contributed to this report.