In his resignation letter, addressed to the Prime Minister, Streeting writes that he has lost confidence in the party's leadership and that it would be "dishonest and unprincipled" to remain as minister.
"Where we needed a vision, we have a vacuum. Where we need leadership, we float free," he writes.
"It is now clear that you will not lead the Labour Party into the next general election."
He does not say whether he intends to challenge Starmer. However, according to previous reports in the British media, Streeting has prepared his resignation as minister in order to mount a serious challenge for the Labour leadership.
However, sources close to Streeting tell Sky News that the former health minister has no intention of challenging Starmer at this time.
Wes has sufficient support, but has made the decision not to challenge immediately because it would not be right for the party, the source says, urging Starmer to draw up a timeline to allow the widest possible election.
Hard pressed
Demands for Keir Starmer's resignation have been mounting in recent days following the Social Democratic Party's major defeat in regional and local elections - in addition to the crisis of confidence surrounding Starmer's appointment of former minister Peter Mandelson, who turned out to have links to the late convicted sex offender Jeffrey Epstein.
Also waiting in the wings is Angela Rayner, who until last year was Starmer's deputy prime minister. Rayner, who belongs to Labour's left wing, is believed to have enough support to run in a party leadership election.
The seemingly most popular candidate, Andy Burnham, is the mayor of Manchester and does not sit in Parliament - a formal requirement for becoming leader. According to several media outlets, Afzal Khan, who is the MP for a Manchester constituency, has agreed to give up his seat for Burnham, who would then have to win a by-election later this year.
Aiming for a leadership role
Streeting, 43, has made no secret of his ambitions to one day take on the role of Labour leader. At the same time, he has been cautious during Starmer's crisis of confidence, repeatedly denying that he intends to challenge the Prime Minister.
The outgoing health minister is considered a strong public speaker and has regularly been sent out to defend the government in the press, according to the BBC. Within the Labour Party, he has gathered support mainly from the centre and right. He was a prominent critic of the party's left-wing swing under Jeremy Corbyn and campaigned to remain in the EU in the Brexit referendum.
