Disastrous results for the governing Labour Party in local elections have plunged Britain into its latest crisis, just under two years after Starmer won a large majority on a vow to bring stability and end a decade of political chaos.
After days of calls by a growing number of Labour MPs for Starmer to either resign or set out a timetable for his departure, Streeting was the first senior minister to break cover, saying he was standing down because "it is now clear you will not lead the Labour Party into the next general election".
"It is now clear ... that Labour MPs (members of parliament) and Labour (trade) unions want the debate about what comes next to be a battle of ideas, not of personalities or petty factionalism," he wrote in his resignation letter.
"It needs to be broad, and it needs the best possible field of candidates. I support that approach and I hope that you will facilitate this."
His announcement on Thursday fell short of triggering a formal leadership contest against Starmer but piles the pressure on the British leader who has so far weathered a drip feed of demands for him to step down.
Sources close to Starmer, who has repeatedly said he would battle to keep his job, say he is determined to fight any leadership contest, which could see him challenged by Streeting and also senior ministers on the left of the party.
Earlier on Thursday, Starmer's former deputy, Angela Rayner, announced she had been cleared of deliberate wrongdoing over her tax affairs, an impediment to any leadership contest, but she would not say whether she wanted to launch a formal bid.
Other potential candidates from the so-called "soft left" of the party - who favour more state involvement in key industries and are pro-workers' rights, with several having close ties to trade unions - include Greater Manchester mayor Andy Burnham and Ed Miliband, the minister for energy security and net zero.
Starmer is not without his supporters.
The 63-year-old former lawyer has adopted a "business as usual" approach and on Thursday his finance minister, Rachel Reeves, warned MPs against "plunging the country into chaos" at a time when Britain's anaemic economy was turning a corner.
She told the BBC that waiting lists for Britain's public health service were falling because of government investment.
"If we put that at risk, we put at risk the investment in our public services and also the growth that is necessary to help people with the cost of living," she said.
While the number of calls for him to resign ebbed on Wednesday, when his government turned to King Charles to set out its agenda for a new parliamentary term, Thursday was yet another showdown for Britain's prime minister.
Rayner, 46, said she had been cleared of tax avoidance, a move she described as exonerating her "of the accusation that I deliberately sought to avoid tax", and paving the way for a potential leadership bid.
But she refused to signal whether she would stand in a contest.
"I'll play my part in doing everything we possibly can to deliver the change, because it's not a personal ambition, I know the difference it makes," she told the Guardian newspaper.
The prospect of another leadership race to choose what would be Britain's seventh prime minister in about a decade has angered business leaders who warn it will deter investment - something the Labour government has said must improve to turn around the nation's fortunes.
