Donald Tusk has said an extension to Article 50 is only possible on the condition that MPs pass the Withdrawal Agreement next week.
The European Council President said he believed European leaders would agree to a “short extension” of Article 50.
He said further negotiation was needed on the duration, and suggested EU leaders would not be satisfied with Theresa May's June 30 proposal.
Mr Tusk said: "In the light of the consultations that I have conducted over the past days, I believe that a short extension would be possible.
"But it would be conditional on a positive vote on the Withdrawal Agreement in the House of Commons.
"The question remains open as to the duration of such an extension."
'Extension - what for?'
Emmanuel Macron is said to believe Brexit is an impediment to his proposed EU reforms.
Mr Tusk said the EU would continue to react with “patience and goodwill” throughout the process.
However he acknowledged that “Brexit fatigue” was “increasingly visible and justified”.
Brexit fatigue is reportedly visible amongst senior French officials, who have suggested that Emmanuel Macron will veto the extension request.
The French Europe minister warned the UK that “time was not the solution.”
She said: “Grant an extension - what for? Time is not a solution, it's a method."
"If there is an objective and a strategy and it has to come from London."