"Today, on behalf of the 27 leaders I can say that we are determined to keep our unity as 27," Tusk said.
EU leaders, including British Prime Minister David Cameron, are scheduled to meet on June 28 in Brussels. Tusk said the 27 leaders of countries remaining in the EU would meet informally to discuss how to handle Britain's decision to leave.
"I have offered the leaders an informal meeting of the 27 in the margins of the European Council summit," Tusk said. "And I will also propose to the leaders that we start a wider reflection on the future of our Union."
Under EU law, a country that wants to leave the bloc has two years for negotiations on the terms of the divorce, starting from the moment it formally notifies the EU of its intention.
Some British officials who campaigned to leave the EU have suggested that London could delay such notification, to make time for informal talks on the best possible exit deal. Tusk appeared to take a firm line.
"All the procedures for the withdrawal of the UK from the EU are clear and set out in the treaties," Tusk said.
Cameron said on June 24 Britain would probably make such a notification only in October.
