The Prime Minister has triggered Article 50 after her letter to European Council President Donald Tusk was delivered by Ambassador Sir Tim Barrow in Brussels.
Under the terms of the Lisbon Treaty the UK will now leave the EU two years from today, and a tough period of negotiations begins with the EU27, nine months on from the referendum vote to leave.
After marking out hard lines on future payments to the European Union and the influence of EU law after Brexit, the text of the Prime Minister’s letter is expected to offer some room for manoeuvre in negotiations.
Theresa May struck a note of conciliation, refusing some calls from her Brexiteer backbenchers for an immediate “cut off” of rights such as permanent residence for new EU migrants.
However, the European Commission, Council and Parliament have signalled that “EU law must apply until Brexit day” – when Britain officially leaves.
The European Parliament, which can veto an UK-EU deal, is next week expected to make the guarantee of full EU citizen rights a red line.
The EU’s chief Brexit negotiator Michel Barnier met in Valletta with the Maltese PM Joseph Muscat, the current holder of the EU presidency. He said it was “day one of a very long and difficult road”.
The Commission also chose the morning of the Article 50 declaration to formally block a key merger between the London Stock exchange and its German counterpart the Deutsche Borse.
Some influential commentators believe this decision was a key test of the EU27s attitude to negotiations with the UK.