Britain’s Labour Party swept to power on July 5 after more than a decade in opposition, as a jaded electorate handed the party a landslide victory — but also a mammoth task of reinvigorating a stagnant economy and dispirited nation.

Labour leader Keir Starmer will officially become prime minister later in the day, leading his party back to government less than five years after it suffered its worst defeat in almost a century.

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In the merciless choreography of British politics, he will take charge in 10 Downing St. shortly after Conservative leader Rishi Sunak and his family left the official residence and King Charles III accepted his resignation at Buckingham Palace.

“This is a difficult day, but I leave this job honored to have been prime minister of the best country in the world,” Sunak said in his farewell address.

Mr. Sunak had conceded defeat earlier in the morning, saying the voters had delivered a “sobering verdict.”

In a magnanimous farewell speech in the same place where he had called for the snap election six weeks earlier, Sunak wished Starmer all the best but also acknowledged his missteps.

“I have heard your anger, your disappointment, and I take responsibility for this loss,” Sunak said. “To all the Conservative candidates and campaigners who worked tirelessly but without success, I’m sorry that we could not deliver what your efforts deserved.”

With almost all the results in, Labour had won 410 seats in the 650-seat House of Commons and the Conservatives 118.

“A mandate like this comes with a great responsibility,” Mr. Starmer acknowledged in a speech to supporters, saying the fight to regain people’s trust after years of disillusionment “is the battle that defines our age.”

Speaking as dawn broke in London, he said Labour would offer “the sunlight of hope, pale at first but getting stronger through the day.”

For Mr. Starmer, it’s a massive triumph that will bring huge challenges, as he faces a weary electorate impatient for change against a gloomy backdrop of economic malaise, mounting distrust in institutions and a fraying social fabric.

“Nothing has gone well in the last 14 years,” said London voter James Erskine, who was optimistic for change in the hours before polls closed. “I just see this as the potential for a seismic shift, and that’s what I’m hoping for.”

And that’s what Mr. Starmer promised, saying “change begins now.”

Anand Menon, professor of European Politics and Foreign Affairs at King’s College London, said British voters were about to see a marked change in political atmosphere from the tumultuous “politics as pantomime” of the last few years.

“I think we’re going to have to get used again to relatively stable government, with ministers staying in power for quite a long time, and with government being able to think beyond the very short term to medium-term objectives,” he said.

Britain has experienced a run of turbulent years — some of it of the Conservatives’ own making and some of it not — that has left many voters pessimistic about their country’s future. The U.K. divorce from the European Union followed by the COVID-19 pandemic and Russia’s invasion of Ukraine battered the economy, while lockdown-breaching parties held by then-Prime Minister Boris Johnson and his staff caused widespread anger.

Rising poverty, crumbling infrastructure and overstretched National Health Service have led to gripes about “Broken Britain.”

Mr. Johnson’s successor, Liz Truss, rocked the economy further with a package of drastic tax cuts and lasted just 49 days in office. Truss, who lost her seat to Labour, was one of a slew of senior Tories kicked out in a stark electoral reckoning.

While the result appears to buck recent rightward electoral shifts in Europe, including in France and Italy, many of those same populist undercurrents flow in Britain. Reform UK leader Nigel Farage roiled the race with his party’s anti-immigrant “take our country back” sentiment and undercut support for the Conservatives and even grabbed some voters from Labour.

The result is a catastrophe for the Conservatives as voters punished them for 14 years of presiding over austerity, Brexit, a pandemic, political scandals and internecine conflict.

The historic defeat — the smallest number of seats in the party’s two-century history — leaves it depleted and in disarray and will spark an immediate contest to replace Sunak, who said he would step down as leader.

In a sign of the volatile public mood and anger at the system, the incoming Parliament will be more fractured and ideologically diverse than any for years. Smaller parties picked up millions of votes, including the centrist Liberal Democrats and Farage’s Reform UK. It won four seats, including one for Farage in the seaside town of Clacton-on-Sea, securing a place in Parliament on his eighth attempt.

The Liberal Democrats won about 70 seats, on a slightly lower share of the vote than Reform because its votes were more efficiently distributed. In Britain’s first-past-the-post system, the candidate with the most votes in each constituency wins.

The Green Party won four seats, up from just one before the election.

One of the biggest losers was the Scottish National Party, which held most of Scotland’s 57 seats before the election but looked set to lose all but handful, mostly to Labour.

Labour did not set pulses racing with its pledges to get the sluggish economy growing, invest in infrastructure and make Britain a “clean energy superpower.”

But the party’s cautious, safety-first campaign delivered the desired result. The party won the support of large chunks of the business community and endorsements from traditionally conservative newspapers, including the Rupert Murdoch-owned Sun tabloid, which praised Starmer for “dragging his party back to the center ground of British politics.”

The Conservative campaign, meanwhile, was plagued by gaffes. The campaign got off to an inauspicious start when rain drenched Sunak as he made the announcement outside 10 Downing St. Then, Sunak went home early from commemorations in France marking the 80th anniversary of the D-Day invasion.

Several Conservatives close to Sunak are being investigated over suspicions they used inside information to place bets on the date of the election before it was announced.

In Henley-on-Thames, about 40 miles (65 kilometers) west of London, voters like Patricia Mulcahy, who is retired, sensed the nation was looking for something different. The community, which has long voted Conservative, flipped to the Liberal Democrats this time.

“The younger generation are far more interested in change,’’ Mulcahy said ahead of the results. “But whoever gets in, they’ve got a heck of a job ahead of them. It’s not going to be easy.”