The upcoming Brexit summit is at risk of not going ahead after Angela Merkel reportedly threatened to boycott it if a deal is not reached first.
EU diplomats have said they had been told the German Chancellor is not willing to come to Sunday's summit for any negotiations, meaning a text must be ready beforehand.
The news cast doubt over whether the summit will go ahead after her diplomats suggested she wants negotiations on Mrs May's draft Brexit agreement finalised within 24 to 48 hours.
She is said to be unwilling to negotiate on terms of the agreement at the summit, which will see British Prime Minister Theresa May and EU leaders meeting in Brussels on Sunday.
The diplomats' comments came ahead of crunch talks between Mrs May and European Commission President Jean-Claude Juncker on Wednesday evening.
Speaking after the meeting, Mrs May said she will return to Brussels on Saturday for more talks with Mr Juncker after the meeting failed to yield a final deal.
"I will be returning on Saturday for further meetings, including again with President Juncker, to discuss how we can ensure that we can conclude this process in the way which is in interests in all our people," Mrs May told the BBC.
"There are some further issues that need resolution," she said.
A Commission spokesman said there was "very good progress" made in the hour and a half meeting.
He said: "Very good progress was made in the meeting between President Juncker and Prime Minister Theresa May. Work is continuing."
EU leaders are due to meet for the Summit on Sunday to rubber-stamp the draft agreement, but sticking points still remain.
Spain has said it will vote against if Gibraltar's future isn't considered a bilateral issue between Madrid and London.
Mrs May is also under intense pressure from pro-Brexit and pro-EU British lawmakers opposing the divorce deal.
