It means the UK may not now leave on 29 March as previously planned.
Mrs May says Brexit could be delayed by three months, to 30 June, if MPs back her withdrawal deal in a vote next week.
If they reject her deal again then she says she will seek a longer extension - but any delay has to be agreed by the 27 other EU member states.
A spokesman for the European Commission said extending Article 50, the mechanism taking the UK out of the EU on 29 March, would need the "unanimous agreement" of all states.
And it would be for the leaders of those states "to consider such a request, giving priority to the need to ensure the functioning of the EU institutions and taking into account the reasons for and duration of a possible extension".
Commission President Jean-Claude Juncker was "in constant contact with all leaders," the spokesman added.'...read more>>>...
