European ministers signed off Britain's draft divorce deal on Monday as they launched an intense final week of negotiations on future cross-Channel ties.
"The first, difficult step is done," said Austrian minister for Europe Gernot Blumel, whose country holds the rotating EU presidency, after the ministers met.
Meanwhile, detailed discussions continue on a parallel political statement setting out the bloc's ambitions for future relations with post-Brexit Britain -- and on a possible extension to the transition period.
Britain will leave the Union on March 29 next year, but remain within its single market for a further 21 months as negotiators seek a deal to avoid a potential breakdown in trade between the key economic partners.
If no deal remains in sight within this period, Britain can request a one-off extension. Barnier has suggested that this should expire at the end of 2022, but he admitted that the other 27 member states have yet to sign off on this date.
"I think that during this week we'll have a definitive proposition for this date. The decision will be made jointly between the United Kingdom and the 27," Barnier said.
Neither European member states, who want to protect access to their single market by non-member Britain, nor hardline British Brexiteers, who fear being trapped in a bloc where they don't make the rules, want an endless transition.
Barnier stressed it was Prime Minister Theresa May's government that had requested the extension option and that he had agreed to it to reassure nervous businesses, but warned: "It can't be indefinite. It needs to be decided."
Meanwhile, preparations for Sunday's summit, where May and her 27 colleagues will sign the withdrawal agreement, continue apace.
"A painful week in European politics is starting," Blomel said. "We have the divorce papers on the table. Forty-five years of difficult marriage are coming to an end."
May has said she will be in the city herself this week to see the president of the EU commission, Jean-Claude Juncker, for last-ditch talks on the future relationship.