LONDON — Prime Minister Liz Truss of Britain announced her resignation on Thursday, bringing a swift end to a six-week stint in office that began with a radical experiment in trickle-down economics and descended into financial and political chaos, as most of those policies were reversed.
With her tax-cutting agenda in tatters, her Conservative Party’s lawmakers in revolt and her government in the hands of people who did not support either her or her policies, Ms. Truss, 47, concluded that she could no longer govern. She departs as the shortest-serving prime minister in British history.
“Given the situation, I cannot deliver the mandate on which I was elected by the Conservative Party,” a grim Ms. Truss said, standing on the rain-slicked pavement outside 10 Downing Street, where only 44 days ago she greeted the public as Britain’s new leader.
Ms. Truss said she would remain in office until the party chooses a successor, by the end of next week. That sets off an extraordinarily compressed, unpredictable scramble to replace her in a party that is both demoralized and deeply divided. Among the likely candidates is Boris Johnson, the flamboyant previous prime minister she replaced after he was forced out in a string of scandals.
Only a day after declaring in Parliament, “I’m a fighter, not a quitter,” Ms. Truss bowed out after a hastily scheduled meeting on Thursday with party elders, including Graham Brady, the head of a group of Conservative lawmakers that plays an influential role in selecting the party leader.
It was the most shocking jolt in a week of seismic developments that included the ouster of Ms. Truss’s chancellor of the Exchequer, Kwasi Kwarteng; the bitter departure of the home secretary, Suella Braverman; and a near melee in Parliament on Wednesday night, as cabinet ministers tried to force unruly Tory lawmakers to back the prime minister in a vote on whether to ban hydraulic fracking.
The spectacle dramatized how Ms. Truss — only the third female prime minister, after Margaret Thatcher and Theresa May — had lost control of her party and government.
By then, though, her mandate had already been shredded: her proposals for sweeping, unfunded tax cuts rattled financial markets because of fears they would blow a hole in Britain’s finances.
That sent the pound into a tailspin that left it briefly near parity with the dollar, forced the Bank of England to intervene in bond markets to stave off the collapse of pension funds and sent mortgage interest rates soaring.
The resulting chaos has left Britons frustrated and jaded, with many convinced the country is spinning out of control.
“We are in an economic crisis, a political crisis, a food crisis — an everything crisis,” said Cristian Cretu, a gas engineer on a break from work. “Whoever is going to replace her, I don’t think they will make a difference.”
The opposition Labour Party called for an immediate general election. But under British law, the Conservatives are not required to call one until January 2025.
If enough Conservative lawmakers joined with the opposition, they could force an election, but with the party’s support collapsing in opinion polls, it is in their interests to delay any encounter with the voters. British political convention also allows them to change party leaders — and therefore the prime minister — using their own flexible rule book.
Ms. Truss’s position was already shaky on Monday, when her newly appointed chancellor, Jeremy Hunt, announced that the government would undo the last vestiges of her tax proposals. As Mr. Hunt presented details of the reworked fiscal plan in Parliament, a silent Ms. Truss sat behind, a faraway smile on her face.
For Britain, it is another chapter in the political convulsions that followed its vote to leave the European Union in 2016. The country will soon have its fifth prime minister in six years. Ms. Truss is the third consecutive leader to be deposed by the Conservative Party, also known as the Tory Party, which now appears to have devolved into warring factions and has fallen as many as 33 percentage points behind the opposition Labour Party in polls.
The political upheaval also comes only a month after Britain buried Queen Elizabeth II, who reigned for seven decades and acted as an anchor for the country. Among the queen’s last official duties was greeting Ms. Truss at Balmoral Castle after she had won the party leadership contest. On Thursday, Ms. Truss said she had informed King Charles III of her decision to step down.
The Conservatives announced rules for the new leadership contest, including a minimum threshold of 100 nominations from lawmakers, which will limit the number of candidates to a maximum of three.
From a shortlist of two, selected by lawmakers, Conservative Party members will then vote online to choose the victor, with the goal of avoiding the prolonged, multistage campaign last summer that resulted in Ms. Truss. In fact, the contest might not get that far: if only one candidate passes the threshold of 100 nominations, or if the second-place contender drops out, there will be a decision on Monday.
“In recent leadership contests, they have chosen someone who is manifestly unsuitable for the job,” said Tim Bale, a professor of politics at Queen Mary University of London. “It is unlikely anyone can rescue them electorally, but there are people who can walk into No. 10 and do the job of prime minister intellectually, emotionally and practically.”
Still, the convulsions of recent days have exposed how divided the Conservative Party is, after 12 exhausting years in power, and how difficult it will be for Ms. Truss’s successor to unite it.
Rishi Sunak, a former chancellor who ran against Ms. Truss last summer and warned that her proposals would produce chaos, should be in the pole position, having led the Treasury and performed well under pressure in the leadership campaign. But he lost that contest largely because many party members blamed him for bringing down Mr. Johnson, from whose cabinet he resigned.
“The obvious candidate is Rishi Sunak,” Professor Bale said. “The question is whether they can forgive him. The situation is now so extreme that people might be prepared to forgive him his supposed sins.”
That is far from clear, however, because Mr. Sunak is also distrusted on the right of the party and among hard-core Brexit supporters in Parliament. His leadership would be hard to stomach for some who opposed him, including the business secretary, Jacob Rees-Mogg, who once refused to deny reports that he had described Mr. Sunak’s policies, which included tax increases, as “socialist.”
Supporters of Mr. Johnson, who is reported to be considering a run at his old job, argue that because of his landslide election victory in 2019, he has a mandate to lead without holding another general election. Under the hashtag #bringbackboris, one of his supporters, James Duddridge, wrote on Twitter: “I hope you enjoyed your holiday boss. Time to come back. Few issues at the office that need addressing.”
But restoring him would be highly risky, given the circumstances of his forced resignation in July and the fact that he remains a polarizing figure among voters. Mr. Johnson is also being investigated by a parliamentary committee over whether he misled the House of Commons about parties held in Downing Street that broke pandemic rules.
Even if Mr. Johnson is exonerated, it will remind Britons of the serial scandals that led lawmakers to oust him. And the committee could recommend Mr. Johnson’s expulsion or suspension from Parliament — a sanction that might mean his constituents get a vote on whether to kick him out of Parliament altogether.
The party’s ideological divisions were laid bare by Ms. Braverman in a blistering letter written after she was fired, ostensibly for breaching security regulations in sending a government document on her personal email. She accused Ms. Truss of backtracking on promises and going soft on immigration.
Ms. Braverman’s parting shot illustrated the resistance from people on the right to what they see as the growing influence of Mr. Hunt, a moderate who voted against Brexit and was a supporter and ally of Mr. Sunak. Mr. Hunt, who has run twice for party leader, said he would not be a candidate this time.
Were the Conservatives to allow Downing Street to fall into the hands of another untested candidate, outside the mainstream, like Ms. Braverman or perhaps Kemi Badenoch, who currently serves as the secretary of international trade, there could be renewed instability in the financial markets.
Penny Mordaunt, the leader of the House of Commons who finished third in the contest last summer, appears well placed to straddle the divide. She is a good communicator, but is untested at the top level of government.
Another option might be a candidate with little ideological baggage, like Ben Wallace, the defense secretary, or Grant Shapps, the new home secretary. But Mr. Wallace decided against running earlier this year, saying he did not want the job enough. Mr. Shapps concluded that he did not have the support to win.
Whoever is chosen will inherit a forbidding array of problems, from 10.1 percent inflation and soaring energy prices to labor unrest and the specter of a deep recession. The new leader will have to make cuts to government spending that are likely to be resisted by different coalitions of Conservative lawmakers.
On Monday, Mr. Hunt said the government would end its huge state intervention to cap energy prices in April, replacing it with a still-undefined program that he said would promote energy efficiency. That could prove unpopular, increasing uncertainty for households facing rising gas and electricity prices.
While the government has abandoned Ms. Truss’s tax cuts — in one of the most striking policy reversals in modern British history — the chaos her program unleashed in the markets has left lingering damage. The rise in interest rates has made borrowing more expensive for the government, economists said, which will produce pressure for even deeper spending cuts.
Despite the Conservative Party’s internal feuds, Professor Bale said he believed it was not inherently ungovernable, so long as it makes the right choice. As recent history has shown, the stakes for the party are extremely high.
“The Conservative Party is an incredibly leadership-dominated party,” he said, “which means that if you get the choice of leader wrong, you’re in serious trouble.”
Euan Ward and Isabella Kwai contributed reporting.
