Brexit Secretary David Davis has said he is entering negotiations on the UK’s exit from the EU in a “positive and constructive” frame of mind.
As he began talks in Brussels, he said he was determined to build a “strong and special partnership” with the EU.
Chief EU negotiator Michel Barnier said he wanted to agree key priorities and a timetable for discussions.
Subjects for the negotiations include the status of expats, the UK’s “divorce bill” and the Northern Ireland border.
The UK is set to leave the EU by the end of March 2019 following last year’s referendum vote.
Day one of the negotiations, at the European Commission buildings in Brussels, will be followed by a joint press conference this evening by Mr Davis and the EU’s chief negotiator Michel Barnier, a former French foreign minister and EU commissioner.
The BBC’s Europe editor Katya Adler said the opening session would focus on basic issues of procedure such as how often the two men and their teams will meet and in what order items will be discussed.
Above all, she added, it would be a “trust-building exercise” after all the “mud-throwing” of recent months.
Arriving in Brussels accompanied by a team of British officials, Mr Davis said there would be “challenges” ahead but he believed the two sides could reach an agreement on the terms of the UK’s exit which “works in the best interests of all citizens”.
“We are starting this negotiation in a positive and constructive tone, determined to build a strong and special partnership between ourselves and our European allies and friends for the future.”
Reflecting on the Finsbury Park attack in north London, he added: “In testing times like these, we are reminded of the values and resolve we share with our closest allies in Europe. There is more that unites us than divides us.”
Mr Barnier said a “constructive” opening to negotiations was vital in setting the tone for what he hoped would be an “orderly” process.
“We must first tackle the uncertainties caused by Brexit – first, for citizens but also for the beneficiaries of EU policies and for the impact on borders, in particular Ireland,” he said.