A no-deal Brexit is "more likely than ever before," European Council President Donald Tusk warned EU leaders on Monday, ahead of this week's summit, while calling for "every effort to reach the best agreement possible."

Efforts to close in on a deal have "proven to be more complicated than some may have expected. We should nevertheless remain hopeful and determined," Tusk wrote in his summit invitation letter, noting that there is "good will to continue these talks on both sides."

"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," Tusk said in the letter. "Let us not give up."

EU leaders are due to decide late Wednesday how to take negotiations forward. Ahead of their discussions, British Prime Minister Theresa May has been invited to give her assessment of the situation.