By Christian Edwards, CNN

London (CNN) — Wes Streeting has resigned as Britain’s health secretary, saying Thursday that he has “lost confidence” in Prime Minister Keir Starmer’s leadership and that it would be “dishonourable and unprincipled” to remain in his government.

The move comes after days of speculation about whether Streeting would formally challenge Starmer’s leadership of the Labour Party. In his letter to the prime minister, Streeting announced his resignation from government, but did not say that he was launching a leadership contest.

In order to trigger that contest, Streeting needs to gain the support of one fifth of Labour’s members of parliament (MPs) – at present, 81 lawmakers.

Starmer has been facing a revolt in his Labour Party since it suffered a drubbing in local elections in England and parliament in Scotland and Wales last week, which has spurred nearly 90 Labour lawmakers to publicly call for Starmer to resign. Streeting is the first member of Starmer’s government to resign since the mutiny began.

In his letter, Streeting said last week’s elections had put “nationalists in power in every corner” of the country – referring to the success of Nigel Farage’s hard-right Reform UK party in England, and of Scottish and Welsh nationalist parties – which he said could threaten the breakup of the United Kingdom. He said that progressive voters were “losing faith” in the Labour Party, citing Starmer’s missteps which he said had “left the country not knowing who we are or what we really stand for.”

“Where we need vision, we have a vacuum. Where we need direction, we have drift,” he said.

Streeting claimed it is clear that Starmer will not lead Labour into the next general election, due in 2029. He said he wanted “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.”

Downing Street did not immediately respond to Streeting’s resignation. All week, however, it has insisted that Starmer has no intention of resigning. In a speech Monday, Starmer vowed to stay in post, saying that a change in leadership would plunge Britain back into the “chaos” that flourished under the Conservative Party, which ousted two leaders in the two years before Starmer came to power in a landslide election in 2024.

To Streeting’s allies, he is one of the best communicators in British politics, adding clarity and fizz to Starmer’s government, which has struggled to tell Britain a compelling story about where Labour is taking it. To his critics, he is nakedly ambitious, unprincipled and lacks obvious appeal beyond the world of Westminster. At the last general election, Streeting clung on to his seat in parliament by just 528 votes.

As health secretary, Streeting had been tasked with overhauling Britain’s creaking National Health Service (NHS), which has eaten up a long-increasing share of the country’s gross domestic product (GDP).

His resignation coincides with the publication on Thursday of government data that shows improvements to the NHS under his leadership. NHS waiting lists fell by 110,000 in March, the biggest monthly drop outside of the Covid-19 pandemic since 2008. Streeting said the data meant the government is “on track to achieve the fastest improvement in NHS waiting times in history.”

Perhaps buoyed by this success, Streeting’s resignation may fire the starting gun on the race to replace Starmer as Labour leader and prime minister. Earlier Thursday, Angela Rayner, the former deputy prime minister, announced that she had resolved a dispute with authorities over her failure to pay enough property tax – a scandal that led to her resignation in September. Although neither Streeting nor Rayner have triggered a leadership contest, both are viewed as potential rivals to Starmer.

Unlike Rayner, Streeting hails from the right of the Labour Party. Much of his life has been spent politicking: First as president of the National Union of Students, then as a local councilor, and now as a member of parliament for a borough in East London, near the public housing estate on which he grew up.

Streeting has made no secret of his respect for the government of Tony Blair, who was prime minister while Streeting was a student at the University of Cambridge. Although Streeting briefly left Labour over Blair’s support for the Iraq War, he is said to have channeled “Blairism” in his mission to reform the NHS.

Any leadership bid may be clouded by Streeting’s friendship with Peter Mandelson, the veteran Labour politician who was fired as Britain’s ambassador to Washington over his ties to Jeffrey Epstein, the convicted sex offender. For months, Starmer has been hounded over his decision to appoint Mandelson, despite his well-known ties to Epstein. Streeting may also be tainted by his association to Mandelson.