Prime Minister David Cameron is to step down by October after the UK voted to leave the European Union.
Mr Cameron made the announcement in a statement outside Downing Street after the final result was announced.
He said he would attempt to "steady the ship" over the coming weeks and months.
He had urged the country to vote Remain, warning of economic and security consequences of an exit, but Leave won by 52% to 48%.
England and Wales voted strongly for Brexit, while London, Scotland and Northern Ireland backed staying in.
UKIP leader Nigel Farage hailed it as the UK's "independence day" but the Remain camp called it a "catastrophe".
The pound fell to its lowest level against the dollar since 1985 as the markets reacted to the results.