The process of allocating the European Union’s top jobs has run into difficulties, Donald Tush has indicated.
The European Council president said that while he had been “cautiously optimistic” about making progress, he was now “more cautious than optimistic”.
Theresa May will join the talks aimed at filling the senior positions in the European Union in what is set to be her final trip to Brussels as Prime Minister.
But if the talks fail to make progress she could be forced into attending another European Council summit to settle the issue.
Mr Tusk’s pessimistic assessment followed a round of talks before the summit, including with German Chancellor Angela Merkel and French President Emmanuel Macron.
He said on Twitter: “Last round of consultations on appointments before the start of #EUCO (the European Council).
“Yesterday I was cautiously optimistic. Today I’m more cautious than optimistic.”
Leaders of EU countries were discussing who should take over from Mr Tusk and European Commission president Jean-Claude Juncker.
Mrs May will take part in the discussions, but is not expected to play a major role in the decision.
A Number 10 spokeswoman said: “As long as we’re still a member we will continue to take part in those discussions, but I think at the same time we are respecting President Tusk’s approach to create a package of candidates across all of the jobs and we recognise this is primarily a matter for the 27.”
A new European Council president, European Commission president, European Central Bank president and EU High Representative for Foreign Affairs and Security Policy will be decided in the talks.
It is not clear whether the “Spitzenkandidaten process” – in which political groupings in the European Parliament put forward their choices – will be used again, following its initiation in the 2014 elections.
But the main political groupings in the Parliament have nominated candidates, including German MEP Manfred Weber, for the centre-right European People’s Party (EPP), and former Dutch foreign minister Frans Timmermans, for the Party of European Socialists (PES).
The liberal group has put forward a “Spitzen Team” made up of figures including European Parliament Brexit co-ordinator Guy Verhofstadt and Danish EU Commissioner Margrethe Vestager.
However, Mr Macron is thought to favour appointing the EU’s chief Brexit negotiator, Michel Barnier, to the role, limiting Germany’s power in the bloc which would grow if Mr Weber became Commission president.
The successful candidate will take office on November 1.
The European Council president will be selected from a field of former heads of government and will assume the role on December 1.
EU leaders will also focus on climate, disinformation, the long-term EU budget and external relations at the summit, before discussing the bloc’s top jobs over dinner.