Senior presiding officer and polling station manager Teresa McCurdy collects a ballot box in the ticket office, off the coast of Northern Ireland. Reuters
The contest for the devolved assembly in Belfast could see a pro-Irish nationalist party win for the first time in the troubled history of the British province.
The results, which are expected from Friday, could have huge constitutional implications for the four-nation UK's future, with predicted victors Sinn Fein committed to a vote in the province on reunification with Ireland.
Senior presiding officer and polling station manager Teresa McCurdy carries a ballot box. Reuters
Polls open at 0600 GMT for councils in Scotland, Wales and much of England, with Johnson facing a potentially pivotal mid-term popularity test.
Poor results could reignite simmering discontent within his ruling Conservatives about his leadership, after a string of recent scandals.
The contest for Northern Ireland's power-sharing assembly is set to capture attention, after numerous polls put Sinn Fein ahead.
A University of Liverpool poll reported Tuesday it remained on target to win comfortably with over a quarter of the vote.
The pro-UK Democratic Unionist Party (DUP) and cross-community Alliance Party were tied for second.