Theresa May will urge the country to "come together" as she prepares to trigger the formal process for Britain's divorce from the European Union.
The UK Prime Minister will call for Leave and Remain supporters to put the referendum behind them and make a success of Brexit as the country embarks on a "momentous journey".
Mrs May will chair a meeting of the Cabinet on Wednesday morning as the letter formally invoking Article 50 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.
Within the following 48 hours, the European Commission is expected to issue "draft negotiation guidelines", which will be sent to the 27 remaining states for consultation.
Their leaders will meet on April 29 at an extraordinary European Council summit to agree a mandate for chief negotiator Michel Barnier and clear the way for talks to begin in earnest in May.
The key point of contention as soon as Article 50 is triggered is the order in which different aspects of Brexit are approached.
Effectively, there are two issues to be settled - the terms of Britain's withdrawal from the EU and arrangements for future trade relations.
On top of that is a possible third negotiation on a "transitional arrangement" covering the period between the moment of departure and new trade rules taking effect.
On Thursday, a white paper will be produced on the Great Repeal Bill, the legislation that will turn more than 40 years of EU regulations into domestic laws.
Labour leader Jeremy Corbyn said: "The British people made the decision to leave the European Union and Labour respects that decision.
"Britain is going to change as a result. The question is how.
"The Conservatives want to use Brexit to turn our country into a low wage tax haven.
"Labour is determined to ensure we can rebuild and transform Britain, so no one and no community is left behind.
"It will be a national failure of historic proportions if the Prime Minister comes back from Brussels without having secured protection for jobs and living standards.
"That's why Labour has set the clear priorities of full access to the European market, rights at work and environmental protection.
"And we will hold the Government to account every step of the way."
Mrs May spoke to Mr Tusk and European Commission president Jean-Claude Juncker by telephone on Tuesday evening.
Mr Juncker said his conversation had been "good and constructive".