Labour faces a period of bitter infighting in Bristol, after an intervention from party HQ prompted the resignation of its leader in the city.
The party's governing national executive committee (NEC) blocked a decision by Labour councillors to join the cabinet of newly elected mayor George Ferguson.
The NEC argued it wanted Labour to be an effective voice of opposition in Bristol following the defeat of its candidate Marvin Rees lost the city's first mayoral election.
But Peter Hammond, the Labour leader in Bristol, plunged the local party into turmoil by quitting his position.
"I have never believed that Labour participation in this cabinet would deliver the manifesto upon which we contested the mayoral election but in these changed political circumstances we should at least be able to be at the table to advocate Labour's approach in dealing with the issues facing the people of Bristol in the face of government actions which do not benefit Bristolians," he said in a statement.
"As a consequence it would be wrong of me to continue as leader of Bristol's Labour councillors where clearly I have their confidence but not that of the wider Labour party."
He added: "I wish my period of leadership of Bristol's Labour councillors hadn't ended this way – but comprising your integrity to stay in office only contributes to a justifiable cynicism in the minds of the public to the political process."
Under the terms of the deal offered by Ferguson, who is notorious in Bristol for his eye-catching red trousers, Labour's former council leader Helen Holland was to become deputy mayor.
Hammond had earlier said Labour politicians in the city wanted to acknowledge Ferguson's victory and acknowledge that "the politics of Bristol have changed significantly".
Labour's secretary in Bristol, Dean Chapman, has also resigned, the Bristol 24-7 website reported.
"After further careful consideration and after listening to a wide range of views across Bristol, and the wider Labour party, we have decided not to enter the cabinet," a south-west Labour party spokesperson said.
"We will support the mayor when we agree with his decisions, and we want to be free to offer constructive alternatives when we believe a better solution is available. We will continue as a Labour party to work with the local communities we represent, and to do our best for the people of Bristol."
Representatives of the Conservatives, Liberal Democrats and Green parties are now expected to fill the three cabinet positions.
The unexpected chaos in Bristol at the start of Ferguson's first term in power is bad news for those supporting the city's transition to the new form of local government. Bristol was the only one of ten cities which voted in favour of switching to a directly elected mayor in referenda held in May.
"Bristol will be one of the trailblazers for mayoral government and the new mayor's first six months in office will be critical to the success of the role," Alexandra Jones of the Centre for Cities said.
"Against a challenging economic backdrop, the mayor has an exciting opportunity to support Bristol, already a thriving city, to deliver economic growth that can benefit not just local residents but also the UK as a whole."