President of the EU Council Mr Tusk was updating the parliament on Brexit talks.
And he told MPs UK officials can still stop the withdrawal from the European Union, despite last year’s referendum.
Mr Tusk said: “Ahead of us is still the toughest stress test. If we fail it, the negotiations will end in our defeat.
“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.”
However, Theresa May’s Government hit back insisting Brexit was still going to happen.
Responding to Mr Tusk’s remarks, the PM’s spokesman said: “Brexit is not going to be reversed.”
Many UK citizens were left angry by the Eurocrat’s comments.
Mark J wrote: “Sorry Donald Tusk. You made Denmark vote again. You made Ireland vote again.
“But you won’t get away with it with the UK.”
Huw Gruffydd added: “Donald Tusk tells UK: You can still stop Brexit. We want to leave! Why hasn’t it already happened?”
And Astro Trader said: “We owe you nothing. Au Revoir!”
And Ukip leader Henry Bolton wrote: “So there it is. Brussels sees #Brexit in terms of a contest not cooperation. #UKIP #OutNow”
Also in today’s speech Mr Tusk hit out at Britain for rejecting the “sacrifices” he had offered when David Cameron was prime minister.
The EU president said: “For many months I was building agreement among the 27 in order to give PM Cameron arguments to stay in the EU, even though this meant that each of the 27 had to make a sacrifice.
“Unfortunately this was not enough for the Brits. After Brexit I worked towards creating the unity of the 27 in the negotiations with the UK. We have shown that no one can divide the 27 in these negotiations.”