Opinion polls have suggested that while big business is broadly in favour of staying in the EU, small firms have been evenly split in what looks like a photo-finish with one poll showing "Remain" at 45% and "Leave" 44%, with 11% undecided.
Prime Minister David Cameron yesterday made a last-minute plea to British voters not to give up the "best of both worlds" by voting to remain in the EU.
Leading 'Brexit' backer Boris Johnson, a former London mayor who is widely touted as a future prime minister, insisted the "Leave" campaign was on the brink of victory.
Immigration to Britain, which has risen significantly in recent years, has been a key issue that has seriously divided the country.
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. The weather forecast for polling day is mixed as a rain-swept nation votes.
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, the BBC reported.
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 Electoral Commission estimates a final result "around breakfast time" tomorrow.
Voting timings: From June 23, 2016 at 7:00 am (IST 11:30 am) till 10:00 pm (IST 02:30 am on June 24)
Counting for London and Northwest to start at starts from 00.45 am on June 24 (IST: 05.15 am)
Major outcome starts between 03.30 am (IST 08:00 am) to 04.00 am (08:30 am) on June 24
Final outcome to be declared till 07.00 am (IST: 11.30 am on June 24)
Total number of counting areas: 382
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