Polls open across the UK on Thursday in local and regional elections that could prove to be historic in Northern Ireland and exert more pressure on Prime Minister Boris Johnson.

The elections in Northern Ireland's capital, Belfast, could see a pro-Irish nationalist party win for the first time.

Northern Ireland's results could have huge constitutional implications for the four-nation UK's future, with predicted victors Sinn Fein - the Irish Republican Army's former political wing - committed to a vote in the province on reunification with Ireland.

Polls open at 0600 GMT for councils in Scotland, Wales and much of England, with Boris Johnson facing a potentially pivotal mid-term popularity test.

Poor results could reignite simmering discontent within his ruling Conservatives Party about his leadership, after a series of recent scandals.

A decisive test for Johnson

Johnson, 57, won a landslide 2019 general election victory by vowing to take Britain out of the European Union.

But his position has been put in jeopardy because of anger at revelations of lockdown-breaking parties at his Downing Street office and a cost-of-living crisis.

Heavy losses could revive calls among Tory MPs to trigger an internal contest to oust Johnson as party leader and from power.

On the other hand, the Labour Party is bidding to leapfrog the Conservatives into second place in Scotland, behind the pro-independence Scottish National Party (SNP), and remain the largest party in Wales, where 16 and 17-year-olds are eligible to vote for the first time. 

"Momentous" election

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 the party 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.

Deirdre Heenan, professor of social policy at Ulster University, said there was a feeling the election "really is momentous."

"It will be a sea change if a nationalist becomes first minister," she told AFP.

Sinn Fein has reduced its calls for Irish unity during campaigning, saying it is "not fixated" on a date for a sovereignty poll, focusing instead on the rising cost of living and other local issues.

Party Vice President Michelle O'Neill has insisted voters are "looking towards the future" with pragmatism rather than the dogmatism that has long been the hallmark of Northern Irish politics.

"They're very much looking towards those of us that can work together versus those that don't want to work together," she said.

Bolstering declining fortunes

But O'Neill's DUP rivals have sought to keep the spotlight on possible Irish reunification in the hope of bolstering their declining fortunes.

In February, the DUP's first minister Paul Givan withdrew from the power-sharing government in protest of post-Brexit trade arrangements, prompting its collapse. 

At a final election debate between the five biggest parties, DUP leader Jeffrey Donaldson reiterated the party would not form a new executive unless London ends the trading terms, known as the Northern Ireland Protocol.