Britain and the European Union (EU) kick off a third round of Brexit talks yesterday still sharply divided over what comes first - London's future relationship with the bloc or the costly divorce settlement.

The European Union says there has to be "sufficient progress" in three key areas - EU citizen rights, Northern Ireland's border and the exit bill - before it will turn to post-Brexit arrangements, possibly beginning in October.

Britain says the two strands should be negotiated in parallel, arguing that progress on a free trade deal may even help resolve other sticky issues such as the future EU-U.K. border between Northern Ireland and the Republic. That is a complete no-go for Brussels, with officials saying they expected little progress in bridging a "very big gap" at this week's talks.

They also blamed Britain for a "lack of substance" despite a flurry of position papers they said were strong on aspiration but short on detail.

EU chief negotiator Michel Barnier meanwhile listed on Twitter the EU's own negotiating documents, noting: "EU positions clear and transparent since day one."

Barnier meets his British counterpart David Davis late afternoon for a first exchange, followed by three days of talks and a joint press conference.

Both sides have repeatedly warned that the clock is ticking down to the March 2019 Brexit deadline and that they are the ones doing their best to make progress.

Davis said this week's talks were all about "driving forward the technical discussions across all the issues."

The talks take place amid continued turmoil in Britain, with the opposition Labour Party over the weekend backing a "soft" Brexit whereby the country remains in the EU's customs union and single market for a transition period.

Prime Minister Theresa May has said she wants Britain unequivocally out of both but her position has been crippled since a June election gamble backfired and she lost her parliamentary majority.

EU officials warned last week that the hard-won Northern Ireland peace process could not be used as a bargaining chip.

London's hopeful suggestion that technology could help prevent the border becoming a physical barrier to trade and the peace process was just "a lot of magical thinking," one EU official said.

Britain also said the European Court of Justice could continue to have an indirect influence, softening its position that the EU's top court would not have any say in the country at all.

As for Britain's divorce settlement - estimated at up to 100 billion euros in Brussels but much less at 40 billion according to reports in London - EU officials said the talks were not about fixing a number but about agreeing how to work out the bill.