Theresa May has signed the letter which will trigger Article 50 and begin Britain’s process of leaving the EU.
The PM has been photographed in the government’s cabinet office as she signs the official letter to the president of the European Council, Donald Tusk.
Mrs May will urge the country to "come together", calling for both Leave and Remain supporters to put the referendum behind them and make a success of Brexit as the country embarks on a "momentous journey".
She will chair a meeting of the Cabinet on Wednesday morning as the letter is dispatched to Brussels.
Over the next two years, the terms of the settlement will be thrashed out between Britain and its 27 counterparts.
The PM will tell MPs she will represent "every person in the UK", including EU nationals, when she takes to the negotiating table.
"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.
"We all want to see a Britain that is stronger than it is today.
"We all want a country that is fairer so that everyone has the chance to succeed.
"We all want a nation that is safe and secure for our children and grandchildren.
"We all want to live in a truly global Britain that gets out and builds relationships with old friends and new allies around the world.
"These are the ambitions of this Government's plan for Britain.
"Ambitions that unite us, so that we are no longer defined by the vote we cast, but by our determination to make a success of the result.
"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."
Signed personally by Mrs May, a so-called "wet signature" in Civil Service jargon, the exit letter will be delivered to European Council president Donald Tusk by the British ambassador to the EU, Sir Tim Barrow, at around 12.30pm UK time.
At roughly the same time, the Prime Minister will rise in the House of Commons to make the statement confirming the two-year countdown to Britain's departure from the EU is finally under way.