"I have to be honest about failure wherever I see it," Sarwar tells a news conference, adding that the situation in No 10 "is not good enough"
Chris Mason analysis: Sarwar's intervention is of colossal significance and is prompting a stream of cabinet ministers to publicly stand up for Starmer
Among those backing the PM are Deputy PM David Lammy and Chancellor Rachel Reeves, who says: "With Keir as our prime minister, we are turning the country around"
Earlier, Starmer's director of communications Tim Allan resigned, a day after the PM's chief of staff also quit. Morgan McSweeney left after taking "full responsibility" for advising Starmer to appoint Peter Mandelson as US ambassador
Labour's ex-deputy leader Angela Rayner, who says Starmer has her "full support".
Rayner, who has been touted as a potential successor to the Labour leadership says in a post on X: "I urge all my colleagues to come together, remember our values and put them into practice as a team.
"The Prime Minister has my full support in leading us to that end."
More members of cabinet have now come out in support of Keir Starmer after Scottish Labour leader Anas Sarwar called on him to quit.
Foreign Secretary Yvette Cooper mentions the upcoming Munich Security Conference, which is happening later this week, saying that "at this crucial time" it's important to have Starmer's "leadership not just at home but on the global stage, and we need to keep our focus where it matters, on keeping our country safe".
"This is not the time for the government to turn inwards on itself. We must focus on delivering the change we promised the country," writes Energy Secretary Ed Miliband on X.
Scotland Secretary Douglas Alexander says the prime minister "has recognised not just that lessons have to be learned but also that we change how we do government. He is right about that and has my support".
Labour was elected to "fundamentally change this country", says Culture Secretary Lisa Nandy as she adds that the PM "has my full support as he works in difficult circumstances to deliver" Labour's vision for the UK.
The intervention from Anas Sarwar is of colossal significance – the most senior Labour figure yet, by some margin, to call for Starmer to go.
It is now prompting a stream of cabinet ministers to go on social media and stand up for the prime minister – after rather a while with a lot of them remaining publicly silent.
The question now will be how many of the cabinet do publicly back the PM? And which ones do not. Today could yet have a lot of mileage in it as this story develops hour by hour.