The UK still has time to rethink its bid to leave the European Union, says President of the European Council Donald Tusk, warning that the “toughest stress test” of Brexit negotiations is still to come.

In a statement to the Members of the European Parliament (MEPs), Tusk said it was “up to London” to decide how the negotiations will end, referring UK Prime Minister Theresa May’s threats of leaving the 27-member bloc without a deal.

“It is in fact up to London how this will end: With a good deal, no deal or no Brexit,” he said.

Tusk also hailed EU members’ unity over Brexit and urged countries to remain together during the next segment of talks in order to “protect our common interest.”

“We have managed to build and maintain unity among the 27,” he said in the report, which was about the summit of EU leaders in the Belgian capital Brussels last Thursday. “But ahead of us is still the toughest stress test. If we fail it, the negotiations will end in our defeat.”

Tusk made the comments days after May acknowledged for the first time that Brexit negotiations with the EU had hit “difficulty.”

Speaking at the EU summit last week, May said: “The clear and urgent imperative must be that the dynamic you create enables us to move forward together.”

Leaders of all EU members agreed at the summit that not enough progress had been made on three key issues that the bloc had singled out as a pre-requisite for proceeding to formal trade talks.

May’s government has failed to address EU concerns about the future of EU citizens in the UK, the amount of a divorce bill that EU officials say the UK must pay and the Northern Ireland border.

The government’s chaotic approach to Brexit has angered the Tory government’s opponents in the Labour Party, allowing party leader Jeremy Corbyn to challenge May’s leadership by expressing readiness to take over the negotiations.

During a dinner with EU leaders during the summit, she pleaded with them to help her strike a deal that she can “defend” at home.