Britain began talks with the EU on the conditions of leaving the bloc, according to a statement from negotiators.
David Davis, the minister in charge of Britain's EU exit, said the talks would see the building of a "new, deep and special relationship" with the EU.
Negotiations would be conducted in a "positive and constructive tone", he added.
The first day of the negotiations began at the European Commission offices in Brussels.
The U.K. is due to leave the EU by the end of March 2019 following last summer's referendum vote.
"There is more that unites us than divides us," Davis said. "So while there will undoubtedly be challenging times ahead of us in the negotiations, we will do all that we can to ensure that we deliver a deal that works in the best interest of all citizens."
The EU's chief negotiator, Michel Barnier, a former French foreign minister and EU commissioner, said he wanted to agree key priorities and a timetable for discussions.
"Our objective is clear, we must first tackle uncertainties caused by Brexit," he told reporters. "First for citizens, also for beneficiaries of EU policies and the impact on borders, in particular Ireland."
