Brussels chief Donald Tusk says there's 'little reason to believe' we can sort out Brexit by the end of June.
It came just minutes after MPs approved Theresa May's plan to seek a delay until June 30.
But inviting EU leaders to a crunch summit in Brussels tomorrow, the European Council President said: "In reality, granting such an extension would increase the risk of a rolling series of short extensions and emergency summits, creating new cliff-edge dates.
"This, in turn, would almost certainly overshadow the business of the EU27 in the months ahead.
"The continued uncertainty would also be bad for our businesses and citizens. Finally, if we failed to agree on any next extension, there would be a risk of an accidental no-deal Brexit.
"This is why I believe we should also discuss an alternative, longer extension. One possibility would be a flexible extension, which would last only as long as necessary and no longer than one year, as beyond that date we will need to decide unanimously on some key European projects."
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The Prime Minister was forced to bring forward a motion under legislation pushed by Labour former minister Yvette Cooper, as part of a parliamentary bid to prevent a no-deal Brexit on April 12 - although any extension is subject to the decision of the EU.
It says she must seek an extension until at least June 30 - but she can agree a longer extension without a further vote in the Commons.
Mr Tusk said a flexible extension would "allow to terminate the extension automatically, as soon as both sides have ratified the Withdrawal Agreement".
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He said: "The UK would be free to leave whenever it is ready. And the EU27 would avoid repeated Brexit summits.
"Importantly, a long extension would provide more certainty and predictability by removing the threat of constantly shifting cliff-edge dates.
"Furthermore, in the event of a continued stalemate, such a longer extension would allow the UK to rethink its Brexit strategy."
He added: "As you know, with Brexit there are no easy solutions. Both aforementioned options have their advantages and disadvantages. Therefore, let us discuss them in an open, creative, and constructive way."
