Polling stations will remain open until 10pm tonight with the final result expected tomorrow morning
The first people able to cast their ballot were in Gibraltar where polling stations opened at 6am UK time and will close at 9pm.
Across the UK, polls opened at 7am and will close at 10pm.
On the ballot paper, voters will be asked: "Should the United Kingdom remain a member of the European Union or leave the European Union?"
They will then choose whether they want to "Remain a member of the European Union" or "Leave the European Union" by placing a cross in the appropriate box.
A record number of voters - some 46.5 million - are eligible to take part in the referendum, according to figures from the UK Electoral Commission.
Counting will begin as soon as polls close at 10pm, with results expected tomorrow morning.
The country has been divided into 382 voting areas, each of which will declare its own result.
This is made of 380 local authorities across England, Wales and Scotland. Northern Ireland counts as one area, as does Gibraltar.
The overall result for the whole of the UK will be announced in Manchester, but only when all 382 areas have declared.
You can listen to a special Brexit results programme on Newstalk from 10pm tonight until midnight, with coverage continuing through the night via LBC radio London. Live updates will be available on newstalk.com