An estimated 46,499,537 people are entitled to take part in the vote - a record number for a British poll.
Also read: What is Brexit and why is it important?
Polling stations opened at 7 a.m. and will close at 10 p.m.
The UK on Thursday (June 23) will decide in a referendum whether to stay in the European Union (EU) or not. The registered voters include Britons from England, Wales, Scotland, Northern Ireland and Gibraltar -- a British territory off the southern coast of Spain.
The referendum ballot paper asks the following question: "Should the United Kingdom remain a member of the European Union or leave the European Union?"
The results are expected by early Friday morning.
Meanwhile, David Cameron made one last appeal before the voting began and said: "Don't vote Leave on the basis of things that aren't true."