The European Council president told MEPs that Theresa May was in charge of whether Brexit talks succeed or fail.
Meanwhile, the EU’s chief Brexit negotiator indicated that agreement on a free trade deal with Britain after it leaves the European Union will take years to complete.
Michel Barnier said the discussions would be very different from the first phase of the negotiations on the terms of Britain’s withdrawal and would entail “risks”.
His comments came after Mrs May told MPs there could be no transition period after Britain leaves the EU unless there was agreement on a trade deal.
Updating the European Parliament on last week’s Brussels summit, Mr Tusk stressed his obsession with the unity of the 27 members who will remain in the EU after the UK’s departure.
“Ahead of us is still the toughest stress test. If we fail it, the negotiations will end in our defeat,” he said.
“We must keep our unity regardless of the direction of the talks. The EU will be able to rise to every scenario as long as we are not divided.
“It is in fact up to London how this will end: With a good deal, no deal or no Brexit. But in each of these scenarios we will protect our common interest only by being together.”
Vince Cable, Leader of the Liberal Democrats, said: “No matter what Theresa May says there are still three options on the table. No deal, a deal, or no Brexit.
The EU have confirmed what we have been saying all along, if Brexit looks like a disaster we can call the whole thing off.”