Efforts to unseat British Prime Minister Keir Starmer broke out into open rebellion Thursday, with one potential rival resigning from the Cabinet and another clearing the way for her to enter any future leadership contest.

Health Secretary Wes Streeting became the first senior minister to quit Starmer's Cabinet in what is expected to be a precursor to challenging his leadership. 

"You have shown courage and statesmanship on the world stage — not least in keeping Britain out of the war in Iran," Streeting wrote in a letter. "But where we need vision, we have a vacuum. Where we need direction, we have drift."

"It is now clear that you will not lead the Labour Party into the next general election," he added.

His announcement followed the resignations of four other members of Starmer's government on Tuesday.

Starmer is facing growing pressure to step down after his Labour Party's disastrous results last week in local and regional elections.

Streeting, whose political ambitions have long been known, is considered one of a handful of people who could try to unseat Starmer. 

In his letter, he called attention to progress in improving health services under his leadership, noting that waiting times for National Health Service appointments — one of his signature priorities — had fallen for the fifth straight month. 

Streeting also argued that supporters "want the debate about what comes next to be a battle of ideas, not of personalities or petty factionalism. It needs to be broad, and it needs the best possible field of candidates."

Other potential challengers

Another likely challenger, former Deputy Prime Minister Angela Rayner, said Thursday that she had reached an agreement with tax authorities to clear up questions about her taxes that forced her to leave the Cabinet last September. 

Rayner told the Guardian newspaper that Starmer should "reflect on" his position, adding that she was ready to "play my part" in any leadership election if Streeting were to trigger a contest.

Greater Manchester Mayor Andy Burnham is also widely seen as a potential candidate, though he would have to find a way back into Parliament before he could run. Allies have suggested a sitting member of the House of Commons could resign to make way for Burnham to run in a special election.

Burnham canceled his regular Thursday appearance on a local BBC radio program this week to "prioritize discussions arising from last week's elections."

Under Labour Party rules, any potential challenger to the prime minister would have to have the backing of 81 of the party's 403 members in the House of Commons. More than that number have publicly called on Starmer to quit in recent days.

Streeting comes from the moderate wing of the left-leaning Labour Party, as does Starmer. Rayner is a favorite of many more left-wing voters, calling on the party to do more to boost the minimum wage and raise taxes on the rich.

Other potential candidates may still enter any race for the leadership.

While there is a chance that the current efforts to unseat Starmer will fizzle out, that would probably just delay the crisis for a few months given the level of fragmentation in British politics, said Jonathan Tonge, a professor of politics at the University of Liverpool.

If "a civil war opens up within a Labour Party that's supposed to be governing us at present, it's an extraordinary state of affairs given it's less than two years since Keir Starmer won one of Labor's greatest election victories ever," Tonge said.

"He's got a huge parliamentary majority, he's got more than 400 MPs, and yet his prime ministership may be on the brink of disintegration," he added.

Economy and election losses take toll 

Pressure for Starmer to step aside has intensified since Labour suffered steep losses in local and regional elections last week, with the populist, anti-immigration party Reform UK, led by Trump ally Nigel Farage, making big gains.

The results underscored voter frustration with a government that has failed to deliver on pledges to boost economic growth and improve living standards for working people.

A stagnant economy and stubbornly high consumer price inflation have made it difficult for Starmer's government to deliver on its promises after winning a landslide election victory less than two years ago.

Starmer has vowed to remain in office, warning lawmakers that any leadership contest would plunge the government into "chaos" at a time it should be focused on issues like the cost of living crisis and war in the Middle East.

"The country expects us to get on with governing," Starmer said Tuesday. "That is what I am doing and what we must do as a Cabinet."

His effort to fight off a leadership challenge was bolstered Thursday morning by a rare bit of positive economic news. Gross domestic product, a broad measure of economic activity, grew 0.6% in the first three months of the year, compared with 0.2% in the previous quarter, the Office for National Statistics said.

Treasury chief Rachel Reeves said the figures showed that her policies were working and that renewed economic growth would allow the government to put more money into public services and programs to support those hit by the high cost of living.

"But that is only possible because of the economic stability that we have brought back to our economy," she told the BBC. "And we shouldn't put that at risk by plunging the country in chaos at a time when there is conflict in the world."