Shadow cabinet member Andy McDonald has quit Labour's front bench with a scathing attack on Sir Keir Starmer.
In his resignation letter - published in the middle of Labour's party conference in Brighton - the MP said his party leader had made Labour "more divided than ever".
Mr McDonald also accused him of not honouring his pledges to members.
Sir Keir thanked him for his service but said his own focus was on "winning the next general election".
Mr McDonald previously served as shadow transport secretary on Jeremy Corbyn's frontbench - but he became one of the few members to survive the handover of power to Sir Keir, and stayed in the top team as shadow secretary for employment rights and protections.
In recent months, his focus had been on creating Labour's new programme of employment rights, which was unveiled at the conference on Saturday by deputy leader Angela Rayner.
'Closing the door'
But earlier this week, former shadow chancellor John McDonnell said the work had been overshadowed by Sir Keir's controversial party rule changes.
Mr McDonald said he had accepted the job "because I wanted to fight for the working people of this country", but, he wrote: "It has become clear to me that I cannot do this as a member of the shadow cabinet."
Sources close to Sir Keir told the BBC they were not unhappy at the departure, insisting this week was "all about change and closing the door on the Corbyn era".
Labour MP Barry Gardiner - who served alongside Mr McDonald in Mr Corbyn's shadow cabinet - told the BBC he was "extremely shocked" to hear of the resignation, saying he had been "a very powerful voice for workers in the country".
But the founder of left-wing Labour activist group Momentum, Jon Lansman, joined Mr McDonald's criticism of Sir Keir, telling BBC News: "He promised to unite the party and actually, unfortunately, he's driving wedges within the party."
Shadow work and pensions secretary Jonathan Reynolds said Mr McDonald "is a friend of mine" and he was "sad to see him go".
But he disagreed with the remarks about division in the party, adding: "We have seen this conference, frankly, Labour facing towards the general election in a way that is not just ambitious in terms of the pledges… but we have also tried to do that in a gain that gains credibility that is required to win power".
As news of the resignation broke in the conference hall, one delegate shouted, "Andy McDonald, solidarity!" to loud cheers from some parts of the crowd.
This is a zinger of a resignation letter, in its content and its timing.
Yes, a man you may not have heard of resigns from a job you probably didn't know he had.
But listen to what Andy McDonald, a man at Labour's top table until today, says about Labour's candidate for prime minister and the movement he leads.
He claims Labour is "more divided than ever" and Sir Keir has broken the promises he made to get elected as party leader, suggesting directly that he can't be trusted.
Political critiques within a political party are often articulated obliquely, with euphemism and understatement.
There is none of this here and it appears slap bang in the middle of the one week of the year where so many of Labour's faithful are in the same postcode.
It will demand an immediate response from Sir Keir Starmer.
In Mr McDonald's letter, the MP claimed his position was "untenable" after the leader's office instructed him go to a meeting at Labour's party conference and "argue against a national minimum wage of £15 an hour and against statutory sick pay at the living wage".
He said it was "something I could not do", adding: "After many months of a pandemic when we made commitments to stand by key workers, I cannot now look those same workers in the eye and tell them they are not worth a wage that is enough to live on, or that they don't deserve security when they are ill.
Mr McDonald said it was a "bitter blow" that Labour had not followed the country in its "renewed awareness of how important the work done by millions of low-paid workers truly is".
He added: "I joined your frontbench team on the basis of the pledges that you made in the leadership campaign to bring about unity within the party and maintain our commitment to socialist policies.
"After 18 months of your leadership, our movement is more divided than ever and the pledges that you made to the membership."
Responding to the letter, Sir Keir released a statement which said: "I want to thank Andy for his service in the shadow cabinet.
"Labour's comprehensive New Deal for Working People shows the scale of our ambition and where our priorities lie.
"My focus and that of the whole party is on winning the next general election so we can deliver for working people who need a Labour government."
Source: BBC | Picture: Getty Images