Brexit primer: the once-in-a-generation European Union referendum explained
There is no precedent for the stay-or-go referendum on the United Kingdom's place in the European Union.
Results from the once-in-a-generation vote are expected to trickle in from about midnight UK time on Thursday [Friday NZ time] after around 46 million people at home and abroad cast votes to remain as part of the 28-nation bloc or go it alone.
If the United Kingdom leaves the EU, with its own parliament and complex system of economic and trade regulations, the divorce could send shockwaves across the European political sphere and the global economy.
Polls have fluctuated, to say the least, and the latest guesstimates put the remain camp slightly ahead of the leave voters, but it's really anybody's game.
* Brexit: 'Leave' camp 10 points ahead in opinion poll
* Analysis: What to know about the Brexit
* IMF says Brexit could deal blow to global economy
* Winston Peters backs Brexit: 'Be bold, ditch the EU'
* What a Brexit would mean for New Zealand
Here's what you need to know, because if the UK does leave the ramifications could affect global financial markets, housing prices and trade between the UK, Europe and the rest of the world including New Zealand and Australia.
And out is out, so the UK cannot have it both ways. In other words, there is no turning around and re-negotiating.
What's the latest?
After four months of campaigning, the debate has polarised the country, the press, pundits and pollsters.
Broadly, there is agreement between experts that leaving could have serious and far-reaching economic consequences.
The debate was stalled and shocked last week when Labour party MP Jo Cox was fatally attacked near Leeds in northern England.
As for the potential impact on countries outside the European economic zone, such as New Zealand, the implications of a Brexit are far from clear. The only real analysis so far is a report by the New Zealand Institute of Economic Research.
The institute warned a Brexit could strengthen a "fortress Britain" mentality and a post-Brexit landscape in a Kiwi context was "horribly murky."
What's going on?
A referendum will be held on Thursday in the UK, polling runs from 7am to 10pm and the first results are expected around midnight.
What's a referendum?
A basic "yes or no" in which almost all eligible British voters, including overseas residents, can cast a vote to stay or remain as part of the EU bloc. Quite simply, voters will be asked whether the UK should remain a member, or leave.
Why the vote?
The British prime minister, David Cameron, vowed to hold a referendum if he was successful in the 2015 general election as a response to growing calls for a political say in the UK's place in Europe.
British, Irish and Commonwealth citizens resident in the UK, plus citizens of Cyprus and Malta.
What is the EU?
A political, legal and economic alliance, formed in 1957 (in a different form to its current incarnation) with the aim of preventing any more bloodshed in Europe after two world wars. The United Kingdom joined in 1973 and the current bloc was constituted as the EU in 1993.
There are currently 28 countries and more than 500 million people in the alliance. Trading in a "single market" means products, goods, services and people can freely move around the continent.
A single currency, the Euro, is used in 19 countries.
The bloc has its own parliament in Brussels, bank and legal framework and no independent European country has ever left the union before.
Who's for and against?
Conservative MP and former London mayor Boris Johnson is one of the best known Brexit advocates. Broadly, the Brexit camp is on the centre right or right of the political spectrum in Britain,
British Prime Minister David Cameron wants the country to remain.
The "remain" camp, the "in" group, argue the economic risk of leaving is considerable, complicated trade deals would need to be renegotiated and the withdrawal could trigger an economic slowdown in the UK, a recession or some kind of economic domino effect.
They say there are many advantages to the economic alliance and the sheer size of the EU, the ease of travel on the continent, the trade benefits of membership, and the young, eager, skilled immigrants who boost Britain's growth.
Britain's opposition Labour Party believes the country should stick with the EU, and so do the International Monetary Fund, the Bank of England and the World Bank.
The economic arguments are, broadly, in agreement, saying the consequences are too uncertain to take the risk.
Cameron has described leaving as a "self destruct option" after a painful post-recession recovery.
"It would be like surviving a fall and then running straight back to the cliff edge," he said.
Those who want to leave the EU, the "Brexiteers", claim the UK has been sidelined by the bloc and has used migration and open borders as an argument for reclaiming sovereignty.
They say the bloc holds Britain back, the EU is overly bureaucratic and its rules cost billions and are often daft. Aspects of the leave campaign have come under heavy fire for anti-immigration sentiment.
UKIP (United Kingdom Independence Party) leader Nigel Farage was blasted for a campaign poster showing a line of refugees and the slogan "Breaking point."
The leave camp want the drawbridge pulled up, they want fewer immigrants, and they express horror at the notion of a "United States of Europe."
Johnson has called for British courage: "This is a moment to be brave, to reach out - not to hug the skirts of Nurse in Brussels, and refer all decisions to someone else".
He's also criticised Cameron's Brexit arguments.
What else do I need to know?
Well, David Cameron has already negotiated a deal of sorts if the voters decide to stay. But the critics say this agreement isn't a guarantee and the in or out option provides certainty.
Cameron hashed out changes to welfare payments for migrants, protection for the financial services run from London and a commitment to greater British self-governing in a European context.
What do the public think?
Polls suggest voters are roughly evenly split, but the numbers are notoriously difficult to rely on and the likely outcome changes depending on the pollster.
What are the consequences of leaving?
That's the billion dollar question.
Economically, the research agrees there is likely to be a slowdown in the UK economy, with ripple effects across Europe and around the world. But, crucially, no-one really knows because a messy divorce on such a macro-economic scale has never been done before.
It also depends on one's view of the EU and whether membership offers economic alliance, freedom of trade and travel, or economic and cultural sidelining.
A win for the leave campaign probably means a new British prime minister, shockwaves in the British economy and uncertainty for financial markets around the world.
Cameron's political destiny, too, will be shaped by the Brexit decision. A vote for Britain to leave the EU will most likely see him out of a job in days.
His arguments in favour of Britain remaining in the bloc have become increasingly strident as the campaign progresses - and his opponents increasingly resentful.
A substantial vote to stay in the EU, by contrast, would bolster Cameron's political credentials and burnish his legacy.
If Britain votes to leave the EU on June 23, it's likely the rumblings across Europe will get louder and more difficult to ignore, and may ignite the exit push in other nations.
There are also concerns about relations between the UK and Ireland, as leaving would likely mean the re-introduction of some kind of permanent border between the Republic of Ireland and Northern Ireland, a backward step after two decades of relative peace.