Boris Johnson is planning to hail “the dawn of a new era” in an address to the nation, one hour before Brexit.
Also today, the prime minister will hold a cabinet meeting in Sunderland – the first city to back Brexit in June 2016’s EU referendum, where more than 60% voted Leave.
Official government celebrations include Downing Street being lit up with a light show, union flags flying in Parliament Square, and three million Brexit-themed 50p coins entering circulation.
Mr Johnson will remain inside 10 Downing Street throughout the evening, while the Brexit Party leader Nigel Farage leads a raucous party beneath the flags in Parliament Square.
But, despite a high-profile campaign, Big Ben will remain silent as the clock strikes 11pm.
It is 47 years since the UK joined the European Economic Community – and tonight, those bonds will be broken.
In Brussels, the UK flag will be removed from its place on the buildings housing the city’s EU institutions, and one union flag is expected to be consigned to a museum.
“A moment of real national renewal”
In his video message, to be released at 10pm, Mr Johnson has promised that he will bring the country together.
He says: “Our job as the government – my job – is to bring this country together and take us forward.
“And the most important thing to say tonight is that this is not an end but a beginning.
“This is the moment when the dawn breaks and the curtain goes up on a new act. It is a moment of real national renewal and change.
“This is the dawn of a new era in which we no longer accept that your life chances – your family’s life chances – should depend on which part of the country you grow up in.
“This is the moment when we begin to unite and level up.”
Brexit: Three years on
The historic moment comes more than three-and-a-half years after the UK voted to Leave – and follows two general elections and three prime ministers: David Cameron, Theresa May and Boris Johnson.
There have been months of meetings, mass protests bringing UK cities to a standstill and endless debates, government defeats and deadlock in parliament, until Mr Johnson won his 80-seat majority in last month’s general election.
Coinciding with the cabinet meeting in Sunderland, the Wearside city’s major landmarks – including a bridge and a lighthouse – are being lit up in red, white and blue to mark the UK’s exit from the EU.
City council leader Graeme Miller said: “Whether you voted Leave or Remain, this is a momentous date in our nation’s history and it is only right that we mark it.
“What is essential now is that we move on from the divisions that have preoccupied the UK since the referendum in 2016 and work together to carve out a new future for our country, and new trade agreements with our EU and other potential trading partners.”
But in Scotland, which voted to stay in the EU in the 2016 referendum, Leave A Light On candlelit vigils are planned in Aberdeen, Dundee, Glasgow and Stirling – aimed at sending a message to the EU to keep open a place for Scotland.
Corbyn: “We will hold government to account”
In his statement to mark Brexit day, the outgoing Labour leader Jeremy Corbyn warned: “Britain is now at a crossroads.
“As we leave the EU, we have an opportunity to shape our future role within the international community for decades to come.
“Britain’s place in the world will change. The question is what direction we now take.
“We can work with other countries to develop trade, improve rights and protections, invest in our communities, tackle the threat of climate catastrophe, fight human rights abuses and promote peace.
“We can build a truly internationalist, diverse and outward-looking Britain.
“Or we can turn inwards, and trade our principles, rights and standards to secure hastily arranged, one-sided, race-to-the-bottom trade deals with Donald Trump and others.”
On Labour’s approach, Mr Corbyn said: “We will hold the government to account every step of the way: to ensure jobs and living standards, rights at work, and consumer and environmental standards are protected as part of whatever is negotiated with the EU, the US or any other country.
“We are determined to defend the gains we have made with others on these issues in the EU. And we will resist a toxic Trump deal that puts our NHS, food standards and jobs at risk.”