Open this photo in gallery Left to right: Former Brexit Party MEPs Annunziata Rees-Mogg, Lance Forman, Lucy Harris and John Longworth in Westminster, central London, on Dec. 5, 2019. The four senior figures quit the Brexit Party on Thursday and urged supporters to vote for the Conservatives in the Dec. 12 election. DANIEL LEAL-OLIVAS/AFP/Getty Images

Just a few months ago Britain’s arch Brexiter, Nigel Farage, was riding high in the opinion polls and his nascent Brexit Party was threatening to reshape the country’s political landscape. But the shine has suddenly come off amid fierce party infighting and a botched election strategy.

On Thursday, Mr. Farage suffered a major blow when four senior figures quit and urged supporters to vote for the Conservatives in the Dec. 12 election. The group included Annunziata Rees-Mogg, one of the party’s few stars and the sister of prominent Conservative cabinet minister Jacob Rees-Mogg.

She and the others had joined the Brexit Party last spring shortly after its launch and they were among the party’s 29 candidates who swept to victory in elections to the European Parliament in May.

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On Thursday, they turned their backs on Mr. Farage and accused him of threatening Brexit. They said his decision to contest more than 275 ridings in the coming election risked splitting the pro-Brexit vote and preventing Prime Minister Boris Johnson’s Conservatives from winning a majority and pursuing a Brexit deal with the European Union.

The group said Mr. Johnson’s agreement wasn’t perfect but it was better to back the Tories than to run the risk of a hung Parliament, which could result in Britain never leaving the EU.

“In Scotland, Wales and England, the Brexit Party are permitting votes to go away from the Conservatives, providing us with a remain coalition that will do anything not to honour the [2016] Brexit referendum,” Ms. Rees-Mogg said during a news conference in London. “I find it absolutely unbelievable but tragic that the Brexit Party, with so many wonderful people dedicated to a cause, are now the very party risking Brexit.”

Mr. Farage has long campaigned for Britain to leave the EU and he played a key role in the 2016 referendum that saw 52 per cent of voters back leaving the bloc. He quit active politics after the referendum but returned to launch the Brexit Party in April after becoming frustrated with then-prime minister Theresa May’s inability to pull Britain out of the EU.

The party enjoyed initial success, winning the most British seats in the European parliamentary election and garnering 32 per cent of the popular vote. Mr. Farage was among those who won a seat.

The party has lost momentum since last summer, when Mr. Johnson replaced Ms. May as Conservative Party Leader and Prime Minister. He took a much harder line on Brexit and managed to secure a withdrawal agreement with the EU in October.

When he couldn’t get the deal through Parliament, Mr. Johnson triggered an election last month and he has campaigned relentlessly on the notion that only the Conservatives can “get Brexit done.”

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Mr. Farage has bitterly opposed Mr. Johnson’s Brexit deal, arguing that Britain should leave the EU without a deal and then negotiate a trade pact. He planned to run Brexit Party candidates in all 650 ridings but changed course after facing criticism they would split the pro-Brexit vote.

He tried to form an election pact with the Conservatives, which would have seen each party stand aside in certain ridings. When the Tories refused, Mr. Farage pulled the party’s candidates in 317 ridings held by the Conservative MPs and he urged voters to elect a block of Brexit Party MPs to keep Mr. Johnson on track.

On Thursday, Mr. Farage defended his election strategy and criticized the rebels as closet Tories. Party officials also accused the group of undermining Mr. Farage and defecting to the Tories in return for plum appointments. The rebels denied any contact with the Conservatives.

“I’m a businessman and a pragmatist and I know when it’s time to take the cake out of the oven,” said John Longworth, a member of the group. “Boris Johnson’s deal is not perfect. However, it has the potential to be a great deal, and there’s every possibility, in the negotiations going forward, that that’s what the outcome will be."

Ms. Rees-Mogg added that she found the suggestion that her brother influenced her decision to quit the Brexit Party “disturbingly old-fashioned … We have completely independent views of each other. I am only concerned about Brexit.”

Mr. Farage has vowed to press ahead with his strategy and he claimed the party was making inroads in Labour Party ridings that voted to leave the EU. There’s little indication he will have much success. Support for the party has sunk to 3 per cent in some opinion polls.

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