Image copyright PA Voters are to head to the polls in a series of elections across the UK on what has been dubbed "Super Thursday".
Elections are taking place for the Scottish Parliament, National Assembly of Wales, the Northern Ireland Assembly and for 124 councils in England.
New mayors will be elected in London, Bristol, Liverpool and Salford, with UK parliamentary by-elections held in Ogmore and Sheffield Brightside.
Police and crime commissioners are also being elected in England and Wales.
How the BBC reports polling day
Polling stations across the length and breadth of the country will open at 07:00 BST, with voting allowed until 22:00 BST.
Thursday's polls are the single largest test of political opinion before the next general election, which is scheduled for 2020.
Elections to the Holyrood Parliament in Edinburgh and the devolved assemblies in Cardiff and Belfast last took place in 2011.
The SNP has been in power in Scotland since 2007, while Labour has run the Welsh government since 1999.
There is a power-sharing arrangement in place in the Northern Ireland Executive. The DUP and Sinn Fein have been the two largest parties at Stormont since 2011.
Election coverage on the BBC
- Follow all the latest updates on the BBC's live page
- Watch Election 2016 on BBC One, hosted by Huw Edwards, from 23:45 BST on Thursday
- Listen to Radio 4 and Radio 5Live's coverage from 00:30 BST on Friday, presented by James Naughtie and Anna Foster
In total, 2,747 seats in English councils - spanning metropolitan boroughs, unitary authorities and district councils - are up for grabs. The majority of the seats were last contested in 2012.
In London, the Greater London Assembly will be elected, as will a successor to the capital's mayor Boris Johnson, who has run City Hall since 2008. Mayoral contests will also be held in Bristol, Liverpool and Salford.
New MPs for the constituencies of Ogmore, in the Vale of Glamorgan, and Sheffield Brightside and Hillsborough will also be chosen.
Image copyright PAThe Ogmore by-election was triggered by former Labour MP Huw Irranca-Davies's decision to stand for the Welsh Assembly. The Sheffield contest was caused by the death of sitting Labour MP Harry Harpham.
Voters in 40 police force areas in England and Wales, excluding London and Greater Manchester, will also elect a police and crime commissioner. The inaugural elections for the positions were held in 2012.
Counting will take place throughout Friday with the majority of results declared on the day.
However, some English council results will not be announced until Saturday while the final result of the Northern Irish election is not expected until Sunday.