Polls show the vote too close to call on "Brexit," the possible British exit from the European Union.
The public, politicians, and the media have been split on the issue, with top British political figures speaking out on both sides.
British Prime Minister David Cameron favors staying in the EU.
"We are a sovereign country. We choose to join NATO. We choose to join the EU," Cameron said. "If we choose to leave, we can leave. But let's be clear. If we do leave, that's it."
"We are walking out the door. We're quitting," he continued. "We're giving up on this organization, which even if we leave, will have a huge effect on our lives, on our children, on our opportunities, on our businesses."
"I don't think that Britain at the end is a quitter. I think we stay and fight. That's what we should do," Cameron concluded.
Former London Mayor Boris Johnson says Britain needs to take back control of its country.
"Do you think we can vote 'leave' and take back control? I think we can," Johnson said. "And of course there are a lot of important things at stake. It's not just lots of money – which is very important – about 10 billion pounds a year that we'll get back net, taking back control of our immigration system, but also of course taking back control of our democracy."
Some opponents have warned that leaving the EU could hurt Britain's economy, but others say getting out could help it.
"I am praying that the UK will leave the EU. If the UK leaves the EU, there will be a movement of other countries that also leave the EU. The breakdown will produce wonders for economic growth," the Economic Times quoted Swiss investor Marc Faber.
Some leaders and analysts have warned that if Britain does leave, it could have extremely serious consequences for the European Union's future.
The United Kingdom has been a member of the European Union since 1973.