Prime Minister Boris Johnson will hail the “dawn of a new era” later, as the UK prepares to leave the European Union after 47 years.
In a video message to be shown at 22:00 GMT – an hour before the official departure time – he will say Brexit is “not an end but a beginning”.
He will describe severing ties with the other 27 EU nations as “a moment of real national renewal and change”.
Little will change immediately, as the UK begins a “transition period”.
Most EU laws will continue to be in force – including the free movement of people – until the end of December, by which time the UK aims to have reached a permanent free trade agreement with the EU.
Labour leader Jeremy Corbyn urged the country not to “turn inwards” and instead “build a truly internationalist, diverse and outward-looking Britain”.
The Archbishop of Canterbury, Justin Welby, told BBC Radio 4’s Today programme: “We must be united in a common vision for our country, however great our differences on achieving it – a common hope for what we want to happen, and what we want to do in the years to come.”
Former Prime Minister David Cameron, who led the campaign to remain in the EU during the 2016 referendum, called it a “very big day for our country”, adding that he believed the UK could “make a success of the choice that we made”.
And Brexit Party leader Nigel Farage said: “At last the day comes when we break free. A massive victory for the people against the establishment.”
Brexit was originally scheduled for 29 March last year but was repeatedly delayed when MPs rejected a previous withdrawal agreement reached by the EU and former Prime Minister Theresa May.
Mr Johnson was able to get his own deal through Parliament after winning December’s general election with a House of Commons majority of 80, on a pledge to “get Brexit done”.
This brought to an end more than three years of political wrangling, following the referendum, in which 52% of voters backed leaving the EU.
The prime minister will hold a cabinet meeting in Sunderland – the city that was the first to back Brexit when results were announced after the 2016 referendum – on Friday morning.
Mr Johnson, who led the 2016 campaign to get the UK out of the EU, will attempt to strike an optimistic, non-triumphalist note in his message, stressing the need to bring all sides together.
“The most important thing to say tonight is that this is not an end but a beginning,” he will say.
“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.”
In his message, filmed in Downing Street, Mr Johnson will also say: “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.”
