Donald Tusk has suggested that European leaders will only approve Theresa May's request for a three month extension if MPs approve her Brexit deal next week.
In a move that significantly raises the stakes, the European Council President said that while he believed a short extension "will be possible", it would be "conditional on a positive vote" on the withdrawal agreement.
His intervention, which comes just hours after May submitted a formal request for Brexit to be delayed until June 30, effectively provides MPs with a binary choice of the Prime Minister's deal or a no-deal divorce, the Daily Telegraph reports.
Speaking in Brussels, Tusk said that should there be no breakthrough he would "not hesitate" to call an emergency meeting next week.
However, without a majority of MPs behind the deal, there would be no guarantee of an extension, meaning the default position would remain the UK leaving the EU on March 29 without an agreement.
In a short statement, Tusk said: "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.
"A question remains open as to the duration of such an extension.
"At this time I don't foresee an extraordinary EU Council. If the leaders approve my recommendations and if there's a positive vote in the House of Commons next week we can finalise and formalise the decision on extension in the written procedure.
"However, if there's such a need I will not hesitate to invite the members of the EU Council for a meeting to Brussels next week."
May suggested she would quit rather than delaying Brexit beyond June 30, telling MPs that Parliament had "indulged itself on Europe for too long".
The length of the extension requested would avert the need to take part in the EU election, although Tusk claimed that it still "created a series of questions of a legal and political nature".
Tonight, May and the leaders of the 27 member states will arrive in Brussels for a European Council summit, where the details of any extension are expected to be provisionally agreed.