"We must prepare the EU for a no-deal scenario, which is more likely than ever before… 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 … Let us not give up," Tusk wrote in an invitation letter to the members of the European Council ahead of their meetings on October 17-18.
"Later … we will decide on how to take the negotiations forward, on the basis of a recommendation by our chief negotiator, Michel Barnier," he said.
Tusk stressed that after the summit in Salzburg, the European Union wanted to see maximum progress leading to a deal in October.
"As things stand today, it has proven to be more complicated than some may have expected. We should nevertheless remain hopeful and determined," he wrote.
UK and EU negotiators met Sunday, in a last-ditch bid to clinch a withdrawal deal ahead of the European Council meeting, but said several issues remained unresolved.
The leaders of 27 remaining EU nations will be briefed on the progress of EU-UK talks by EU chief Brexit negotiator Michel Barnier over dinner on Wednesday evening.
In June 2016, the United Kingdom voted to leave the European Union. Although Brexit is scheduled for late March 2019, London and Brussels still cannot agree on a number of key issues, including the Irish border and customs arrangements, making a no-deal scenario a possibility.