Officials from both sides are meeting in Brussels, but chief EU negotiator Michel Barnier and British counterpart David Davis are not attending the start, underscoring low expectations for the talks.
European Commission spokesperson Margaritis Schinas meanwhile challenged an assertion May was due to make in parliament that "the ball is in their court" and that the EU needed to show flexibility.
Britain says there will be no Brexit bill figure by October
"This is not exactly a ball game," Schinas told a press conference.
"We do not provide comment on comments but what I can remind you of is there is a clear sequencing to these talks and there has been so far no solution found on step one, which is the divorce proceedings," he said.
"So the ball is entirely in the UK court for the rest to happen," he added.
The talks have stalled on all three of the key divorce issues -- the exit bill Britain must pay, the rights of EU citizens living in Britain and the fate of the border between Northern Ireland and Ireland.
Juncker says 'wind is back in Europe's sails'
This fifth round of divorce discussions is the last before European leaders meet at a summit on October 19 to decide whether there is "sufficient progress" to move on to the trade talks that Britain desperately wants.
The prognosis is grim, with European Commission chief Jean-Claude Juncker having warned that "miracles" are needed this week to make enough progress to get a positive decision at the summit.
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