Prime Minister Boris Johnson's Conservative party lost control of key councils in London, according to partial results from local and regional UK elections on Friday, with a potentially historic change looming in Northern Ireland.

The main UK opposition Labour party of Keir Starmer won in long-term Conservative strongholds in the capital, including Margaret Thatcher's "favourite" council Wandsworth, and Westminster for the first time since it was created in 1964.

Just over half of votes for councils in England have been counted. Results from the remainder, as well as Scotland, Wales and Northern Ireland are expected throughout the day.

The contest for the devolved assembly in Belfast could see a pro-Irish nationalist party win for the first time, with huge constitutional implications for the four-nation UK.

Predicted victors Sinn Feinthe former political wing of the paramilitary group the IRAare committed to a vote on reunification with the Irish republic to the south, a century after the island was partitioned.

The English results so far are not a landslide for Labour, which is seeking to capitalise on a cost-of-living crisis and Johnson's own performance, including his unprecedented police fine for attending a lockdown-breaking party at Downing Street.

Johnson called the results so far "mixed" and said he took responsibility.

"We had a tough night in some parts of the country but on the other hand in other parts of the country you are still seeing Conservatives going forward," Johnson told reporters in his constituency on the outskirts of London.

But Starmer, visiting Barnet in northwest London, where Labour seized control of the council from the Tories, hailed what he called "a big turning point".

"When it comes to London, you can hardly believe those names come off our lips. Wandsworth! They've been saying for years 'You'll never take Wandsworth from us.' We've just done it! Westminster! It's an astonishing result," he told supporters.

'Catastrophic'

The former chief of staff of Conservative ex-premier Theresa May said the results in London were "catastrophic".

"Wandsworth and Westminster were flagship councils," Gavin Barwell tweeted.

"Losing them should be a wake-up call for the Conservative Party."

The Conservatives are hoping to extend their 12 years in power for another term at the next general election, which is due by 2024.

Johnson, 57, won a landslide 2019 general election victory by vowing to take the UK out of the European Union, and reverse rampant regional inequality.

Despite making good on his Brexit pledge, the coronavirus pandemic largely stalled his domestic plans.

But his position has been put in jeopardy because of anger at lockdown-breaking parties at his Downing Street office and the steeply rising cost of living.

Poor results could reignite questions about his leadership, putting his position in jeopardy.

Labour 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 16and 17-year-olds are eligible to vote for the first time.

SNP leader Nicola Sturgeon hailed a "seismic" result after her party took the only Tory seat in Glasgow.

Sinn Fein rise

The contest for the 90 seats in Northern Ireland's power-sharing assembly is set to capture attention after numerous polls put Sinn Fein ahead.

The pro-UK Democratic Unionist Party (DUP) and cross-community Alliance Party were tied for second.

First results were expected in the mid-afternoon but the full count was likely to go late into the night, or resume on Saturday if it proves particularly tight for the more hard-fought seats, an election official said.

Early indications were that turnout was about 54 per cent down from nearly 65 per cent in 2017.

Sinn Fein has dialled down its calls for Irish unity during campaigning, saying it is "not fixated" on a date for a sovereignty poll, instead of focusing 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.

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

In February, its first minister withdrew from the power-sharing government in protest at post-Brexit trade arrangements, prompting its collapse.

DUP leader Jeffrey Donaldson has said that his party would not form a new executive unless London rips up the trading terms, known as the Northern Ireland Protocol.