British Prime Minister Boris Johnson will hail Brexit day as "the dawn of a new era" and pledge to unite Britain in an address to the nation later today.
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".
"Our job as the government — my job — is to bring this country together and take us forward," Johnson will say, according to extracts released by his office.
"And the most important thing to say tonight is that this is not an end but a beginning ... It is a moment of real national renewal and change," Reuters cited him as saying.
Brexit was originally scheduled for March 29, last year but was repeatedly delayed when MPs rejected a previous withdrawal agreement reached by the EU and former Prime Minister Theresa May.
