Donald Tusk, President of the European Council, has tweeted a photo of the Article 50 letter being given to him by UK envoy Sir Tim Barrow.
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.
Theresa May takes Prime Minister's Questions in the Commons after which she will give a statement on Article 50.
The £21 billion merger between the London Stock Exchange and Deutsche Borse has been blocked by European regulators.
The European Commission said the two exchanges had failed to address competition concerns.
Margrethe Vestager, the EU's competition commissioner, said the merger would have "significantly reduced competition by creating a de facto monopoly".
"As the parties failed to offer the remedies required to address our competition concerns, the Commission has decided to prohibit the merger."
Chancellor Philip Hammond has hailed today as "the moment when we put behind us the divisions and questions of the past" and focus on getting "the very best possible arrangement" with the European Union.
As Britain prepares to trigger Article 50 to begin the Brexit process, Mr Hammond said: "We're going to get the very best deal that we can for Britain, and we're going to deliver the crucial things which I think people were asking for - control over migration and a reassertion of the supremacy of our Parliament."
ITV News' Political Editor Robert Peston has said the triggering of Article 50 is reason for excitement and anxiety.
He compared Britain leaving the EU to his "beloved Arsenal" hypothetically saying it would leave the Premier League to join a global super-league.
"I'd feel excited by that prospect, but also a little bit fearful that it might not all work out as we hoped," he said.
He added that the other risk is Scotland might want to leave the UK, which could fracture the union.
"It's an imperfect analogy and all I really wanted to say is that of course on a day like today, one feels excitement about what this wonderful world has to offer, but it's also a day one might feel slightly anxious," Peston said.