On Tuesday afternoon, the Prime Minister signed the letter that starts the formal exit process. Tomorrow, the document will be hand-delivered by a senior diplomat to EU chiefs.
Once it has been accepted, Article 50 has been officially launched.
On the eve of that historic handover, Ms May urged the country to come together.
“When I sit around the negotiating table in the months ahead, I will represent every person in the whole United Kingdom – young and old, rich and poor, city, town, country and all the villages and hamlets in between,” she said.
“And yes, those EU nationals who have made this country their home.
“It is my fierce determination to get the right deal for every single person in this country.”
She said her guiding principles would be ensuring the UK was stronger and fairer than it is today.
Ms My also repeated her mantra about creating a “truly global Britain” that “builds relationships with old friends and new allies around the world”.
She concluded: “We are one great union of people and nations with a proud history and a bright future.
“And, now that the decision has been made to leave the EU, it is time to come together.”
The PM's top team will gather around the Cabinet table at No 10 on Wednesday morning as she informs them about the content of the letter formally invoking Article 50 of the Lisbon Treaty – the formal notification of Britain’s intention to leave the EU.
Then, at some point after 12.30pm, Ms May will inform MPs that Brexit is being triggered and in Brussels, British ambassador to the EU Sir Tim Barrow will deliver the document to European Council president Donald Tusk.
London mayor Sadiq Khan seized on her promise to defend the interests of EU citizens living in the country.
"I'm pleased that the Prime Minister has today recognised the importance of reassuring EU citizens living in Britain who are understandably extremely concerned about their future,” he said.
"Today, Theresa May has a huge opportunity to give them a cast-iron guarantee that they can stay here after Brexit as she triggers Article 50.
"This would start negotiations with a powerful symbol of goodwill and both sides should give this assurance today."
Meanwhile, three former cabinet ministers have warned of the catastrophic consequences of a so-called hard Brexit.
Tory Lord Heseltine branded the move a “nightmare”, Labour’s Lord Mandelson said it was equivalent to “shooting ourselves in both feet”, and Liberal Democrat Sir Vince Cable said the fall out would be as disruptive as the credit crunch.