The Prime Minister has now signed the letter formally starting the UK's withdrawal from the European Union.
Giving official notice under Article 50 of the Lisbon Treaty, it will be delivered to European Council President Donald Tusk tomorrow.
Theresa May will tell MPs in the Commons that this will mark "the moment for the country to come together".
A Downing Street spokesperson said: "In separate calls, they agreed that a strong EU was in everyone's interests and that the UK would remain a close and committed ally."
Theresa May called the German Chancellor, the President of the European Council, and the President of the European Commission to update them, ahead of her letter triggering Article 50 and kicking off Brexit negotiations, being delivered tomorrow.
“They also agreed on the importance of entering into negotiations in a constructive and positive spirit, and of ensuring a smooth and orderly exit process," Number 10 said. "The Chancellor, the European Council President, and the European Commission President thanked the Prime Minister for her calls."
The UK’s ambassador to the EU, Sir Tim Barrow, will hand-deliver the notification letter to the European Council.
From the notification date the UK will then have two years to negotiate the terms of exit. If this process is followed correctly the UK will have left the EU officially by the start of April 2019.