The by-election, which takes place outside the normal parliamentary election cycle, was triggered by the resignation of a Labour lawmaker in March and is one of dozens of votes that took place on Thursday.
Britain's ruling Conservative Party has won a new seat in parliament, sweeping aside the incumbent Labour Party with a victory in the northern English town of Hartlepool that tightens its grip on traditional Labour-voting areas.
Conservative candidate Jill Mortimer beat the Labour candidate by 15,529 votes to 8,589, securing what less than a decade ago would have been seen as an impossible feat of dislodging the main opposition Labour Party from one of its heartland seats.
The overwhelming victory, which gives Prime Minister Boris Johnson an even larger majority in parliament, increases pressure on Labour leader Keir Starmer, who has faced criticism for not fulfilling his pledge to revive the party's fortunes after a 2019 election disaster.
"There's no hiding from the fact this is a shattering result for Labour, absolutely shattering," Labour lawmaker Steve Reed, a member of Starmer's top team, told the BBC.
"It tells us that the pace of change in the Labour Party has not been fast enough. We need to quicken it up."
The Conservatives described it as a historic day.
Biggest swing of votes
The so-called by-election, which takes place outside the normal parliamentary election cycle, was triggered by the resignation of a Labour lawmaker in March, and is one of dozens of votes which took place on Thursday.
Election analysts said it was the biggest swing of votes to the governing party at a by-election since World War Two.
"It's quite a spectacular turnaround in a seat that Labour should really have saved and defended," politics professor Michael Thrasher told Sky News.
Winning Hartlepool further boosts Johnson and his strategy, particularly after some feared a furore over the financing of a lavish refurbishment of his Downing Street apartment could dent his popularity around the country.
An election watchdog is investigating the financing of the refurbishment, which Johnson says he paid for and in doing so, followed all the rules.
Scotland's independence may be back on table
Thursday's other votes range from votes for local councils to those for parliaments in Scotland and Wales, gauging support for Britain's two main parties and in Scotland's case, the depth of backing for its leading party's push for independence.
Results for the Edinburgh parliament are expected on Saturday evening, with the Scottish National Party (SNP) seeking a pro-independence majority to put pressure on Johnson to permit another referendum on splitting from the UK .
Johnson has said Scotland's 2014 referendum, when 55 percent of people voted "no" to independence, closed the debate for a generation.
"This is not the time to have a reckless and I think, irresponsible, referendum," he said, pushing back against SNP leader and Scottish First Minister Nicola Sturgeon, who wants another vote once the pandemic subsides.
The results of the other contests will come out over several days because Covid-19 restrictions have complicated the counts.
Crushing defeat for Labour in Hartlepool. Not possible to blame Jeremy Corbyn for this result. Labour won the seat twice under his leadership. Keir Starmer must think again about his strategy #HartlepoolByElection— Diane Abbott MP (@HackneyAbbott) May 7, 2021
Chipping the red wall
The Hartlepool result continues a trend set by Johnson in the 2019 general election when he struck directly at Labour's heartlands, the so-called "Red Wall" areas of northern and central England, to win a commanding majority in parliament.
Labour had tried to manage expectations over the vote, saying that Thursday's elections would always be difficult at a time of the coronavirus pandemic, which has boosted support for the government because of its rapid vaccine rollout.
But the loss is painful for Starmer, who has struggled to increase his appeal to the electorate despite heavy criticism of the Conservative government over its initial, sluggish response to the Covid crisis and possible conflicts of interest.
Starmer has tried to shift his party towards the centre ground after two defeats under the left-wing leadership of Jeremy Corbyn. But the Hartlepool defeat drew immediate criticism from some members of the party's leftist faction.
"Not possible to blame Jeremy Corbyn for this result," said lawmaker Diane Abbott, a Corbyn ally.
"Keir Starmer must think again about his strategy."