The UK will leave the EU at 23:00 GMT, ending 47 years of membership.
In a video message to be released an hour earlier, Prime Minister Boris Johnson will call Brexit – which follows more than three years of political wrangling – a “new dawn”.
A series of events including marches, celebrations and candlelit vigils will be held by both Brexiteers and pro-EU demonstrators.
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.