Donald Tusk said a no deal Brexit scenario is “more likely than ever before” in a letter to members of the European Council ahead of their upcoming meeting.
Mr Tusk, the council’s president, made the statement alongside a message rallying those involved to “not give up”.
He has invited Prime Minister Theresa May to speak to the EU27, to give the UK Government’s “assessment of the negotiations”.
Detailing his stance on the discussions at present, he said a Brexit deal has “proven to be more complicated than some may have expected”.
Following this, he wrote: “We should nevertheless remain hopeful and determined, as there is good will to continue these talks on both sides.
“But at the same time, responsible as we are, we must prepare the EU for a no-deal scenario, which is more likely than ever before. Like the UK, the Commission has started such preparations, and will give us an update during the meeting.
“But let me be absolutely clear. The fact that we are preparing for a no-deal scenario must not, under any circumstances, lead us away from making every effort to reach the best agreement possible, for all sides.
“This is what our state of mind should be at this stage. As someone rightly said: ‘It always seems impossible until it’s done.’ Let us not give up.”
He said the European Council “will decide on how to take the negotiations forward, on the basis of a recommendation by our chief negotiator, Michel Barnier”.
They are meeting on Wednesday, October 17, and Thursday, October 18.
Theresa May, on Monday, addressed the House of Commons in regards to Brexit.
This was after reports that negotiations with Brexit secretary Dominic Raab and Mr Barnier had come to an impasse.
She told MPs: “I continue to believe that a negotiated deal is the best outcome for the UK and for the European Union.
“I continue to believe that such a deal is achievable.
“And that is the spirit in which I will continue to work with our European partners.”