Fears are growing in Brussels that the chaotic political situation in London after Prime Minister Theresa May suffered a string of ministerial resignations will hamper the chances of reaching a deal on key issues, especially Britain's contentious exit bill.
EU leaders had hoped to officially approve the next phase on future relations and a transition period at a summit in December, but officials are increasingly concerned that deadline could slip to February or March.
"The European Council summit in October wanted to keep up the dynamic of the negotiations and I am of the same state of mind," Barnier said. "But the real moment of clarification is coming."
This week's talks feature a stripped-down two-day schedule instead of the normal four days, with Barnier and his British counterpart David Davis set to meet only tomorrow morning before holding a press conference, sources said.
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To move on to trade talks, the EU is demanding sufficient progress on three key divorce issues, above all the bill Britain must pay to cover its budgetary commitments, a figure which senior European officials put at 60 billion euros (USD 70 billion).
But the British government looks increasingly distracted, with the resignation of its aid minister over meetings in Israel yesterday adding to the sense of chaos since May's disastrous showing in elections earlier this year.
Britain also faces an economic slowdown, with the EU cutting its growth outlook for the country this year and warning that times would remain tough to 2019 as "uncertainty" over Brexit weighs heavily.
"We are a bit concerned about what we are seeing in the UK at the moment, we want a strong negotiating partner," an EU diplomat told AFP on condition of anonymity.
That timescale would allow preparations for a formal decision by EU leaders at a summit on December 14-15 to move on to discussing future ties, a step Britain has been pushing for for months.
"Everything is ready (to start trade talks) on the first of January," the EU diplomat said.
Failure to do so would probably push back the move to one of the next summits in February or March, leaving only around six months to reach a deal by October 2018, the timeline Barnier has set in order for the withdrawal agreement to be ratified by Brexit day in March 2019.
The EU says Britain must provide written guarantees of a pledge to honour the financial commitments that May made in a speech in Florence, Italy, in September.
"We don't need speeches, we need commitments," the diplomat said.
European Parliament chief Antonio Tajani -- whose institution will have the final vote on any Brexit deal -- last month set the bill at 50 to 60 billion euros (USD 58-70 billion), and said that the 20 billion proposed by London was "peanuts".
"We don't recognise reports suggesting that a deal on citizens' rights is almost finalised. There are still major issues that have to be resolved," said the parliament's Brexit steering group, chaired by former Belgian premier Guy Verhofstadt.