Millions of Britons are voting in a historic referendum on whether the UK should remain a member of the European Union or leave.
An estimated 46,499,537 people are entitled to take part in the vote - a record number for a UK poll. Experts have been predicting turnout of between 60 and 80 percent.
More on the referendum from RNZ
Polling stations opened at 7am BST (Thursday 6pm) and will close at 10pm BST (9am today NZT).
Long queues formed at polling stations despite thunderstorms and downpours in London and south-east England which caused flooding overnight. Two polling stations in south west London had to be moved after they were inundated with water.
It is only the third nationwide referendum in UK history and comes after a four-month battle for votes between the Leave and Remain campaigns.
The referendum ballot paper asks the following question: "Should the United Kingdom remain a member of the European Union or leave the European Union?"
Whichever side gets more than half of all votes cast is considered to have won.
Opinion polls taken before the vote suggest that the result still hangs in the balance - with the latest to be released showing the two sides were still neck and neck.
Mr Cameron said the decision would be irreversible and there would no coming back if the UK voted to leave.
"You can't jump out the aeroplane and then clamber back through the cockpit hatch," he told the BBC.
Leaving the EU would be a "massive problem" for the UK, he said, doing "untold damage" to economic growth, jobs and family finances and hindering the opportunities and life chances of future generations.
Mr Johnson and his fellow campaigners, however, said only a vote to leave the EU could give the UK the freedom it needed to set its own course, and rejected economic forecasts suggesting the country would face a downturn following Brexit.
He urged Britons to "believe in our country" and seize the moment.
"It's time to speak up for democracy, and hundreds of millions of people around Europe agree with us. It's time to break away from the failing and dysfunctional EU system."
European Commission chief Jean-Claude Juncker, meanwhile, warned there would be no further renegotiation whatever the result, after EU leaders reached a deal on a new settlement for Britain in February.
Result due today
After the referendum polls close, sealed ballot boxes will be collected and transported to the count venue for each of the 382 local counting areas.
These represent all 380 local government areas in England, Scotland and Wales, plus one each for Northern Ireland and Gibraltar.
Individual areas' results will then be declared throughout the night, along with results from 11 regional counts.
Depending on how close the poll is, the result may become clear before the final national result is officially declared by the Chief Counting Officer, who will be based at Manchester Town Hall.
The UK's electoral commission estimates a final result "around breakfast time" on Friday.
- BBC / Reuters / RNZ