Britain has formally given the EU notification that it intends to leave the bloc.
Sir Tim Barrow, British ambassador to the EU, handed the written letter to European Council president Donald Tusk on Wednesday.
The minute the letter was accepted, the notification to leave was triggered.
Britain now has two years to negotiate an agreement with the 27 remaining EU countries over its future relationship with the bloc.
The triggering of Article 50 has dominated newspaper headlines on Thursday, with many focusing on a possible threat over security by Ms May.
The former head of the Diplomatic Service breaks down the hidden meanings behind Theresa May's letter to Donald Tusk
Nine months on, Stratford - where the referendum vote was 52:48 - remains as divided as ever over whether leaving the EU is a good idea.