The European Union will grant the UK another delay to Brexit with conditions, including holding European Parliament elections, according to draft conclusions of the bloc's national leaders due to decide on the matter tomorrow.
It would cut Britain off on 1 June if it failed to honour that condition.
According to the draft the EU will agree "an extension to allow for the ratification of the Withdrawal Agreement".
It could be cut short if both the EU and the UK ratify the Brexit deal before it runs its course.
The EU will demand that Britain's extended stay in the bloc cannot undermine its key institutions.
"If the UK is still a member of the EU on 23-26 May 2019 and if it has not ratified the Withdrawal Agreement by 22 May 2019, it must hold the elections to the European Parliament in accordance with union law.
"If the United Kingdom fails to live up to this obligation, the withdrawal will take place on 1 June 2019," the draft said.
"The United Kingdom shall facilitate the achievement of the union's tasks and refrain from any measure which could jeopardise the attainment of the union's objectives."
The draft conclusions, however, have for now left the end-date of the Brexit postponement blank before national EU diplomats discuss them.
Several EU diplomats said that the 12-month extension proposed by European Council President Donald Tusk, who will chair the summit tomorrow, was unlikely to fly and several member states have agreed with France that that would be too long.
Instead, they said that the Brexit lag could go until the end of the year but that the 27 national leaders in the bloc would have the final say tomorrow.
Donald Tusk suggests EU leaders grant UK a longer Brexit extension
European Council president Donald Tusk has suggested EU leaders grant the UK a longer extension to Brexit than Theresa May has requested.
The British Prime Minister, who spent the day holding talks with French president Emmanuel Macron in Paris and German Chancellor Angela Merkel in Berlin, has been making the case for a delay until 30 June.
However, in a letter to the heads of the 27 remaining member states ahead of a crunch summit tomorrow, Mr Tusk said there was "little reason to believe" that the ratification of Mrs May's beleaguered Brexit deal could be completed by the end of June.
He called for the European Council to discuss an alternative, longer extension, such as a "flexible extension" lasting "as long as necessary and no longer than one year".
Mr Tusk wrote: "The flexibility would allow to terminate the extension automatically, as soon as both sides have ratified the Withdrawal Agreement.
"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 suggested that the EU would grant an extension rather than allowing Britain to leave without a deal on Friday, saying that, given the "risks posed" for those on both sides of the English Channel, "I trust that we will continue to do our utmost to avoid this scenario".
Additional reporting: PA