Britain starts formal talks to leave the EU on Monday, seeking a deal "like no other in history" despite entering fiendishly difficult negotiations with a badly weakened government.

A year after Britain's seismic referendum, Brexit minister David Davis and the European Union's French chief negotiator Michel Barnier will meet at the European Commission in Brussels.

At stake in hugely complex talks that are expected to conclude by March 2019 is not just Britain's future but a western political order that would be badly shaken by a failure to reach a deal.

But the situation is very different from 12 months ago when the Brexiteers were riding high, with Prime Minister Theresa May's entire approach called into question after a disastrous election performance on June 8.

"While there is a long road ahead, our destination is clear — a deep and special partnership between the UK and the EU. A deal like no other in history," Davis said in a statement as he headed into the talks.

"I look forward to beginning work on that new future."

Britain already appears to have capitulated to the EU's insistence that talks first focus on three key divorce issues, before moving onto the future EU-UK relationship and a possible trade deal.

Those issues are Britain's exit bill, estimated by Brussels at around 100 billion euros ($112 billion), the rights of three million EU nationals living in Britain and one million Britons on the continent, and the status of the border between Northern Ireland and Ireland.

"Sitting down for a first formal negotiation round is something in and of itself," an EU source said.

Talks will begin at 0900 GMT with a joint press conference by former French foreign minister and European commissioner Barnier and Davis at around 1630 GMT.

Worried by immigration and loss of sovereignty, Britain voted last year to end its four-decades-old membership of the 28-country bloc — the first state ever to do so — in a shock referendum result.

An increasingly concerned EU has been pushing London to hurry up, with time running out for a deal and three months already passed since May triggered the two-year Article 50 EU exit process.

Threats by Britain to walk away without a deal have also worried European capitals.

Today's talks however are likely to focus on the practical details of timings for the coming months, with the big, divisive issues left aside for now, officials said.

May herself will also have a chance to update the other 27 EU leaders on her Brexit plans at a summit in Brussels on Thursday.

"The best way we can spend this week is to rebuild trust," another European source said.

Amid reports that May is set to make a "generous offer" on the rights of EU citizens remaining in Britain, the source said London had been warned against doing so this week, on the grounds that it could drag up the thorny issue before talks had really got going.

Yet many in Brussels fear that London has no real strategy, with May under pressure at home, still trying to close a deal with a conservative Northern Ireland party to stay in power, and facing criticism for her handling of the aftermath of a devastating tower block fire.