Theresa May has signed the letter that will formally begin the UK’s departure from the European Union.
Giving official notice under Article 50 of the Lisbon Treaty, it will be delivered to European Council president Donald Tusk later.
In a statement in the Commons, the prime minister will then tell MPs this marks “the moment for the country to come together”.
It follows June’s referendum which resulted in a vote to leave the EU.
Mrs May’s letter will be delivered at 12:30 BST on Wednesday by the British ambassador to the EU, Sir Tim Barrow.
The prime minister, who will chair a cabinet meeting in the morning, will then make a statement to MPs confirming the countdown to the UK’s departure from the EU is under way.
She will promise to “represent every person in the whole United Kingdom” during the negotiations – including EU nationals, whose status after Brexit has yet to be settled.
“It is my fierce determination to get the right deal for every single person in this country,” she will say.
“For, as we face the opportunities ahead of us on this momentous journey, our shared values, interests and ambitions can – and must – bring us together.”
Attempting to move on from the divisions of June’s referendum, Mrs May will add: “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.”
Labour leader Jeremy Corbyn said his party respected the decision to leave the EU and would hold the government to account “every step of the way”.
He said: “Britain is going to change as a result. The question is how.”
According to the BBC, Corbyn warned it would be “a national failure of historic proportions” if Mrs May does not secure protection for workers’ rights.