Prime Minister Theresa May has promised to trigger Article 50 of the Treaty of the European Union -- the divorce notification clause -- by the end of March.
Tusk said the European Council, the EU institution which groups the bloc's national leaders, would issue its draft plan for the talks soon after that.
"When the UK notifies, it is our goal to react with the draft negotiation guidelines for the 27 members to consider, for this I think we need more or less 48 hours," Tusk told a news conference in Brussels.
Article 50 will begin a two-year countdown to Britain's actual exit, during which Britain and the EU must negotiate the terms of the departure and, they hope, a future trade relationship.
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Tusk said he still hoped it was possible for May to trigger Article 50 earlier than the end of March despite the process being held up in the British parliament "but it is not our decision".
"We are well prepared and our reaction after formal notification will be fast and responsible," he said.
Formal talks are however not set to begin until at least two months after the notification as the European Commission, the executive arm of the EU, must also set out more detailed plans for the negotiations based on guidelines from the leaders.