In a dramatic press conference, Sarwar said the situation in Downing Street was "not good enough" and that "too many mistakes" had been made at the top of government.

However, just as Sarwar began to speak, a flurry of senior Labour ministers took to social media to express their support for the prime minister.

Sir Keir continues fighting to shore up his position, after his judgement was questioned over the appointment of Peter Mandelson as UK ambassador to the US, despite his links to convicted sex offender Jeffrey Epstein.

On Monday evening, the prime minister addressed a packed meeting of the Parliamentary Labour Party in the Houses of Parliament, where he was greeted with applause from his MPs.

The prime minister said he would not be stepping down, telling the gathering: "Every fight I've been in, I have won."

He said: "I have had my detractors every step along the way, and I've got them now.

"But I'll tell you this, after having fought so hard for the chance to change our country, I'm not prepared to walk away from my mandate and my responsibility to my country, or to plunge us into chaos, as others have done."

Speaking immediately after the meeting, former minister Justin Madders said he wanted to "hear more of what we heard tonight" from the PM.

"I don't think there was any sense in that room that night that people want a change in prime minister," he said.

However several sources inside the room told the BBC there were a number of critical questions from MPs.

"Several of my colleague were very forthright," one Labour MP told the BBC as they left the room.

At the same time as the PM was addressing MPs, Health Secretary Wes Streeting - who has been touted as a possible leadership rival to Sir Keir - took the unusual step of publishing some of the messages between himself and Lord Mandelson, a move he said was aimed at dispelling a "weekend of smear and innuendo that I have something to hide".

"Contrary to what has been widely reported, I was not a close friend of Peter Mandelson, but I am not going to wash my hands of my actual association with him either," Streeting wrote in the Guardian.

The messages include Streeting criticising the government for having "no [economic] growth strategy".

Although, the day ended with a standing ovation for the prime minister from Labour MPs and endorsements from his cabinet colleagues, that could change quickly and his position remains far from secure.

There have been months of speculation about Sir Keir's leadership, but the row over Lord Mandelson's appointment has intensified discussions of a change at the top.

Around half a dozen Labour backbench MPs have called for the prime minister's resignation and, in the past two days, two senior advisers left No 10.

Announcing his decision to resign as the Downing Street chief of staff, Morgan McSweeney said he took "full responsibility" for advising the prime minister to appoint Lord Mandelson.

Last week, Sarwar criticised the decision to pick Lord Mandelson and on Monday he said it was time for the prime minister to go.

He said his announcement was "not easy and not without pain", adding that Sir Keir was a "decent man" but that his "first loyalty" was to Scotland.

With elections taking place in Scotland in three months' time, Sarwar said he needed to "decide what I'm willing to accept and what I'm willing to tolerate".

Speaking about the government, he said: "They promised they were going to be different, but too much has happened.

"Have there been good things? Of course, there have been many of them, but no one knows them and no one can hear them because they're being drowned out - that's why it cannot continue."

Asked who he wanted to see replace Sir Keir, Sarwar said he was "not supporting or backing any alternative or any candidate".

Watch: The PM's not out the woods, says the BBC's Nick EardleyIn the hour following Sarwar's press conference, a succession of cabinet ministers expressed their support for the prime minister:

Earlier on Monday, Tim Allan became the fourth communications chief to leave Downing Street in less than a year.

In a short statement, he said: "I have decided to stand down to allow a new No 10 team to be built. I wish the PM and his team every success."

Sir Chris Wormald, the cabinet secretary and most senior civil servant in the country, is likely to follow Allan in leaving his job, with an announcement expected in the next couple of days.

It comes after the resignation of McSweeney, who said Lord Mandelson's appointment had been "wrong" and that "in the circumstances, the only honourable course is to step aside".

Sir Keir sacked the former Labour minister and peer in September, when documents were released showing what appeared to be a close relationship between the disgraced financier Epstein and Lord Mandelson.

The prime minister continues to insist he was lied to by Lord Mandelson about the depth and nature of the relationship.

The loss of McSweeney, a man whose advice Sir Keir has relied on since his campaign to become the leader of the Labour party, was already a major blow to his premiership.

Allan's sudden departure, after five months in the job, causes the PM further problems, making it all the more important for Sir Keir to demonstrate to Labour MPs that he has a plan to turn things around.

Conservative leader Kemi Badenoch said the resignation was "yet another sign that Keir Starmer has lost control of his government".

Speaking to broadcasters she said the prime minister was "in office but not in power" and "completely at the mercy of Labour MPs".

Deputy Liberal Democrat leader Daisy Cooper said: "The British public are crying out for a prime minister and a government that will focus on the cost of living, fixing the NHS and social care.

"They don't want to see a prime minister more preoccupied with his own survival while the country pays the price."

Reform UK leader Nigel Farage said he thought the prime minister would have to resign "before very long".

"Anas Sarwar is a very big marker - how can Labour fight the Scottish elections if Starmer stays as prime minister?"

Shortly before Allan resigned, Sir Keir gave a speech to No 10 staff, asking them to help him "prove that politics can be a force for good" through programmes to tackle child poverty, the cost of living crisis and cutting NHS waiting lists.

Referring to the revelations about Lord Mandelson, he said: "The thing that makes me most angry is the undermining of the belief that politics can be a force for good and can change lives."

Sir Keir told staff he had been "absolutely clear" that he regretted his decision to appoint Lord Mandelson and had apologised to the victims, before paying tribute to McSweeney, whom he has worked with for the last eight years.

Thanking a man he referred to as his "friend", he said: "We changed the Labour Party together. We won a general election together. And none of that would have been possible without Morgan McSweeney.

"His dedication, his commitment and his loyalty to our party and our country was second to none."

McSweeney's deputies, Vidhya Alakeson and Jill Cuthbertson, have been appointed joint acting chiefs of staff.

But the prime minister continues to come under sustained criticism.

Labour backbencher Norwich South MP Clive Lewis told Radio 5 Live he did not believe McSweeney's departure would staunch the wound.

"It's about the whole political culture Keir Starmer has ushered into his administration, which makes proximity to wealth and power the project," he said.

A high-profile Labour MP, Dame Emily Thornberry, backed Sir Keir as a "good leader" but said he must "step up a bit more than he has" to ensure clear leadership.

Polly Billington, Labour MP for East Thanet and vice chair of the Parliamentary Labour Party, told the BBC that following the resignations from No 10 she hoped to see "some significant changes in personnel" that would address her concerns about a "boys' club" at the top of government.

"If you surround yourself with a bunch of men, you make bad decisions," she added.

'Sarwar takes biggest gamble of political career'More from the BBC17 mins agoMaxwell refuses to answer questions about Epstein in congressional hearingA video released by members of Congress shows Maxwell, the jailed associate of Jeffrey Epstein, refusing to answer questions about the late sex offender.

Watch: Ghislaine Maxwell repeatedly invokes right to silence during depositionThe convicted sex offender was asked if she was a close friend of Jeffrey Epstein and if she helped him traffic girls - and remained silent.

Mona Juul and her husband Terje Rød-Larsen - who played a vital role in the Oslo Accords - are being investigated for "aggravated corruption".

Making his first intervention in the Epstein scandal, the King said he is ready to support the police in their inquiries.

Eluned Morgan remains tight-lipped on whether the prime minister should stay or go.