LONDON: Britain's departure from the European Union may still not happen, one of the EU's top officials has suggested.

Donald Tusk, President of the European Council, said the onus was on London to ensure the two sides came to a deal before Britain's scheduled departure from the EU in May 2019.

Tusk, updating the European Parliament on Tuesday after a summit of European leaders last week, said it was up to Britain whether there was a "good deal, no deal, or no Brexit" at all.

Tusk warned that the EU would suffer if the talks ended in failure. "Ahead of us is still the toughest stress test. If we fail it, the negotiations will end in our defeat," he told members of the European Parliament.

"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."  Tusk has expressed a hope in the past that the UK would not leave the EU. But there is no suggestion he sees it as a serious possibility.

There is no precedent for revoking Article 50 of the Lisbon Treaty, the legal mechanism for leaving the EU triggered by the British government in April.

In any case, British Prime Minister Theresa May has vowed to deliver Brexit, and the main UK opposition Labour party does not favor reversing the Brexit referendum.