Prime Minister Boris Johnson will hail the “dawn of a new era” on Friday, as the UK prepares to leave the European Union after 47 years.
In a speech 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.
In a statement, Labour leader Jeremy Corbyn urged the country not to “turn inwards” and instead “build a truly internationalist, diverse and outward-looking Britain'”.
Brexit was originally scheduled for 31 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 78, on a pledge to “get Brexit done”.
This brought to an end more than three years of political wrangling, following the referendum of 2016, in which 52% of voters backed leaving the EU.