EU leaders have agreed to move Brexit Talks on to the second Phase but called for "further clarity" from the UK about its future intentions.
The first issue to be discussed, at talks next month, will be the terms of an expected two-year transition phase after the UK's March 2019 exit.
Theresa May welcomed the move as an "important step on the road" to ensuring a "smooth and orderly" exit.
But European Commission President Jean-Claude Juncker has warned the negotiations will be "significantly harder" from now on.
Donald Tusk, the president of the European Council, tweeted the news that the 27 EU leaders were happy to move onto phase two after they met in Brussels.
He congratulated UK Prime Minister Theresa May on reaching what the BBC's political editor Laura Kuenssberg said was a major moment in the Brexit process.
Mrs May has said she wants both sides to approach the next phase of negotiations, which will determine the long-term shape of the UK relations with the EU, with "creativity and ambition".