LONDON — Voters weary of economic pain and political turmoil handed Britain’s governing Conservatives two thumping defeats Friday in a trio of special elections that point toward likely defeat for the party in the next national election.

The Conservatives avoided a wipeout by holding onto the seat in suburban London of a former premier, Boris Johnson — a sliver of comfort for Prime Minister Sunak’s party.

Mr. Sunak said the results showed that the next general election, due by the end of 2024, was not “a done deal.”

Yet an elections analyst, John Curtice, said the Conservatives were “in a deep electoral hole” after the main opposition Labour Party and the smaller centrist Liberal Democrats overturned huge Conservative majorities to win a seat apiece.

The three results show the Conservatives losing ground across a broad range of voters: suburban Londoners, residents of small towns in the north of England, and rural residents in the southwest. If replicated at a general election, the results would see Labour become the biggest single party, possibly with an overall majority.

“We hear that cry for change away from the chaos, away from those rising bills, the crumbling public services — a cry for change and we will deliver,” the Labour leader, Keir Starmer, said alongside the party’s 25-year-old winning candidate, Keir Mather, at the northern seat of Selby and Ainsty.

“The first time we’ve won here is the first time we’ve overturned a 20,000 majority, the biggest majority we’ve ever turned over in the history of the Labour Party,” Mr. Starmer added.

The Liberal Democrats took the rural seat of Somerton and Frome in southwest England with a similarly large swing away from the Conservatives.

“The people of Somerton and Frome have spoken for the rest of the country who are fed up with Rishi Sunak’s out-of-touch Conservative government,” said Liberal Democrat leader Ed Davey. He congratulated candidate Sarah Dyke beside a confetti-firing mock circus cannon emblazoned “get these clowns out of No. 10,” the prime minister’s 10 Downing St. residence.

The Conservatives won Uxbridge and South Ruislip at west London by 495 votes — down from a majority of 7,000 under Mr. Johnson — after a campaign that focused on an unpopular local green levy imposed by London’s Labour mayor.

Mr. Sunak headed straight to the scene of his party’s sole electoral success and noted that governments often find midterm elections difficult.

“The message I take away is that we’ve got to double down, stick to our plan and deliver for people,” he said during a visit to a cafe in the constituency.

The defeats don’t mean a change of government, since the Conservatives still have a chunky majority in the House of Commons. Yet they confirm the trend of opinion polls, which for months have given Labour a lead of up to 20 percent nationwide over the Conservatives, who have been in power since 2010.

The two defeats also showed electors voting tactically, backing the party most likely to defeat the Conservative candidate. That will leave many Conservative lawmakers rattled ahead of a national vote.

A professor of politics at Queen Mary University of London, Tim Bale, said it was tempting “for Conservative spin doctors to emphasize Uxbridge and the result that they got there. But I think really they will be deluding themselves if they think that that bodes well for the next general election.”

The right-of-center governing party has been plagued by the fallout from the tumultuous terms of Mr. Johnson and his successor, Prime Minister Truss, who quit within weeks after her plan for unfunded tax cuts alarmed financial markets, worsening a cost-of-living crisis and sending mortgage costs soaring.

Mr. Johnson triggered one of the special elections when he quit as a lawmaker last month, almost a year after resigning as prime minister, when a standards watchdog concluded he’d lied to Parliament about lawbreaking parties in his office during the coronavirus pandemic. The former lawmaker at Selby, a Johnson ally, followed him out the door, while the legislator at Somerton resigned amid sex and drugs allegations.

The bruising defeats make it likely that Mr. Sunak will shake up his government with a Cabinet shuffle when Parliament returns from its summer recess in September. Under Britain’s parliamentary system, Mr. Sunak can call a national election whenever he feels the time is right, though it must be within five years of the last one, held in December 2019.

There are also questions for Mr. Starmer, who has been cautious in laying out his plans for government, to the frustration of some Labour supporters.

Labour’s defeat at Uxbridge will likely stoke concern over Mayor Sadiq Khan’s plan to expand an anti-pollution zone first introduced when Mr. Johnson was mayor to outer boroughs of London, slapping a daily emissions charge on older gas and diesel vehicles.

Mr. Starmer acknowledged the levy was the reason Labour lost at Uxbridge. He said, “the mayor needs to reflect. And it’s too early to say what should happen next.”

Mr. Bale said Labour “will worry a little bit, since much of the party’s economic message is built around green issues.”

Yet he said the Conservatives should be more concerned, because many voters who backed them in 2019 had become disillusioned.

“We got used to the idea that somehow there was this big realignment in British politics and the Conservatives were capable of winning seats where they weren’t before,” he said. “I think the Conservatives now should be quite worried ”

Associated Press