Voting is taking place in a historic referendum on whether the UK should remain a member of the European Union or leave.
A record 46,499,537 people are entitled to take part, according to provisional figures from the Electoral Commission.
Polling stations will close at 22:00 BST with counting throughout the night.
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.
Sunshine and heavy showers are forecast for Northern Ireland and Scotland but it is set to be drier and brighter elsewhere.
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.
Results from these areas will then be declared throughout the night, along with result totals from 11 nations and regions.
