Britain will not leave the EU until May 1 next year at the earliest.
Brussels is gearing up to offer another six-month extension when the present October 31 deadline expires.
A decision will be made at a summit on June 20 which EU Council president Donald Tusk describes as a “taking stock” session.
Technical parts of the EU withdrawal agreement are being redrafted to include flexible dates rather than set deadlines.
Brussels now accepts that with a Tory leadership contest over the summer a new PM will not be ready to take Britain out of Europe in October.

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Theresa May planned to give MPs the chance to come to an agreement this week from a selection of Brexit options with a series of indicative votes.
But the PM has been warned that a bizarre alliance of Labour MPs and Tory Jacob Rees-Mogg ’s fanatical Brexiteers will block the move.
She will discuss with Cabinet whether trying to hold the votes is worth the candle anyway.
Mrs May is now working on pulling a rabbit out of the hat to get MPs to support her Withdrawal bill.

(Image: Getty Images)
A Whitehall source said: “We intend to make a bold offer that will allow Parliament to back the bill, get the deal over the line and deliver Brexit.”
The Brexit landscape has changed significantly since Boris Johnson threw his hat in for the Tory leadership on Thursday.
It played a major part in talks collapsing.
Mrs May promised a “Boris Lock” on any compromise reached with Jeremy Corbyn ’s team so it could not be overturned.
But a Labour insider said: “They couldn’t guarantee it wouldn’t be ripped up within 15 minutes of the Tory leadership contest ending. That was the fatal blow.”