Video report by ITV News Political Editor Robert Peston
Theresa May has begun Britain's "momentous journey" to leave the EU by signing the letter that will trigger Article 50.
The historic document, which will formally begin the UK's departure from the EU, will be delivered to the European Council in Brussels by the British ambassador to the EU at 12.30pm on Wednesday.
At roughly the same time, Mrs May will rise in the House of Commons to make the statement confirming the two-year countdown to Britain's departure from the EU is under way.
She is expected to say: "It is my fierce determination to get the right deal for every single person in this country.
"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.
Mrs May spoke to European Council president Donald Tusk and European Commission president Jean-Claude Juncker by telephone on Tuesday evening.
She will chair a meeting of the Cabinet on Wednesday morning before speaking in Parliament.
Within 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.
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.
Over the next two years, the terms of the settlement will be thrashed out between Britain and its 27 counterparts.
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."