Theresa May will issue an appeal to the country to "come together" as she formally launches the process for the UK to leave the European Union.
Nine months after the divisive Brexit referendum last June, the Prime Minister will officially trigger Article 50 in a letter that will be hand delivered in Brussels.
At the same time, in a statement to MPs in the Commons, she will pledge to get the right deal for everyone in the UK, including EU nationals living in this country.
"We are one great union of people and nations with a proud history and a bright future," she will say.
"And, now that the decision has been made to leave the EU, it is time to come together."
While the Prime Minister's approach on the historic day of the triggering of Article 50 will be typically business-like, many Brexit-supporting Conservative MPs are in a mood to celebrate.
Although Tory MPs have been discouraged from holding Article 50 parties, many are preparing to defy the plea and begin celebrating immediately after the PM has finished answering questions on her statement.
There is also a mood of anticipation and excitement in Whitehall. One senior government source, describing the mood ahead of triggering Article 50, said: "It's like Christmas Eve".
The letter invoking Article 50, signed personally by the PM, will be delivered to European Council president Donald Tusk by Britain's EU ambassador Sir Tim Barrow at around 12.30pm UK time.