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Angela Rayner has been one of the governing Labour Party’s most popular figures.Hannah McKay/Reuters

British Prime Minister Keir Starmer is facing a fresh round of political turmoil after his deputy resigned on Friday for failing to pay the proper amount of land tax on the purchase of a new home.

The departure of Deputy Prime Minister Angela Rayner, one of the governing Labour Party’s most popular figures, as Parliament resumed is a huge political blow to Mr. Starmer, whose government was already trailing Reform UK in most opinion polls.

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She was among his most loyal and trusted lieutenants. Along with serving as Deputy Prime Minister, she was Secretary of State for Housing, Communities and Local Government.

Her working-class background as a teen mom who rose through the ranks of the union movement also made her a popular figure among Labour faithful and she was elected Deputy Leader in 2020 when Mr. Starmer was elected leader.

Her resignation “will be a body blow for the Starmer government,” said Victoria Honeyman, a professor of British politics at the University of Leeds. “Angela Rayner was a strong Deputy Prime Minister and minister and a strong voice. She brought an extra voice into the room, one very much needed in discussions of policy, and I think that will be greatly missed.”

Ms. Rayner stepped down from her cabinet roles and as Deputy Party Leader after a review by Independent Ethics Adviser Laurie Magnus found that she did not meet “the highest possible standards of proper conduct” expected by cabinet ministers.

His review had been sparked by weeks of media reports about Ms. Rayner’s purchase of an £800,000, or $1.5-million, home earlier this year and allegations that she failed to pay as much as £40,000 in land tax, known as stamp duty.

Ms. Rayner said she paid for the home largely by selling her interest in a family property she owned with her ex-husband and her young son, who is disabled and whose share was held in trust. She added that she consulted a pair of real-estate lawyers about how much stamp duty to pay.

In his report, Mr. Magnus said the lawyers cautioned her to seek expert tax advice because of the complexities surrounding the trust. She failed to seek that advice, he added, and ended up paying an incorrect amount of tax.

In her resignation letter to Mr. Starmer, Ms. Rayner acknowledged that her actions “did not meet the highest standards.”

“I deeply regret my decision to not seek additional specialist tax advice given both my position as Housing Secretary and my complex family arrangements. I take full responsibility for this error,” she added. “I would like to take this opportunity to repeat that it was never my intention to do anything other than pay the right amount. I must also consider the significant toll that the ongoing pressure of the media is taking on my family.”

Mr. Starmer said he was sorry to see her go. “Although I believe you have reached the right decision, it is a decision which I know is very painful for you,” he wrote in a letter to her. “You have been a trusted colleague and a true friend for many years. I have nothing but admiration for you and huge respect for your achievements in politics.”

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Her resignation derailed Mr. Starmer’s plan to relaunch his government and forced him into a cabinet shuffle. The current Foreign Secretary David Lammy has been named Deputy Prime Minister and Justice Secretary. Yvette Cooper will replace Mr. Lammy as Foreign Secretary and Shabana Mahmood moves from Justice to become Home Secretary.

The Labour Party will also have to hold an election for a new Deputy Leader in the coming months. That could open divisions among Labour MPs as various candidates come forward and challenge government policies.

Mr. Starmer has already confronted fierce opposition from backbench MPs and party members over cuts to social benefits. The Chancellor of the Exchequer, Rachel Reeves, has also been criticized for increasing taxes and failing to do enough to encourage economic growth. She will table the government’s budget in November and it’s widely expected to include further tax increases and spending cuts.

The scandal swirling around Ms. Rayner has also been a boost for Reform leader Nigel Farage. Reform only has four MPs, but it has been topping most opinion polls and winning over high-profile defections from the Conservative Party.

“This government is deep in crisis,” Mr. Farage told the party’s annual conference on Friday in Birmingham. “Not only have they fallen into deep unpopularity in just a year of winning the general election but it’s become clear to all of us that it is a Cabinet full of wholly unqualified people to run our country.”

Polling expert John Curtice, professor of politics at the University of Strathclyde, said that current polling suggests Mr. Farage is potentially on course to become Britain’s next Prime Minister.

“Never before has a party other than Conservative or Labour been ahead in the polls for so long,” he said in an analysis published by the BBC on Friday. “It has consistently been averaging 30 per cent to 31 per cent, enough to put it well ahead of all its rivals.”