Nelson Mandela Bay speaker Jonathan Lawack has been removed after a motion of no confidence against him was passed by the city council with one vote.
NMB city manager Johan Mettler confirmed that 60 councillors had voted in favour of the motion, while 59 voted against it.
READ: Opposition coalition against Trollip gets more time as they bicker over replacement
The new speaker will be elected at 14:00.
Before the meeting, DA councillor Victor Manyati said he had taken a "sober decision" not to vote with his party.
"I need change in this metro. In fact, not only in this metro, in the entire South Africa," he told reporters in Port Elizabeth.
Manyati said he would be leaving the DA.
He abstained during the vote, thereby handing the required majority to the opposition.
In May, Manyati appeared in court on charges of insurance fraud.
The removal of Lawack is the latest incarnation of the turmoil in the NMB council.
Two weeks ago, a bid to oust Mayor Athol Trollip failed as members of the opposition collapsed the meeting. They complained they had not received their council meeting agendas on time.
The ANC, EFF, UDM, AIC and PA announced in early August that they had agreed to work together to remove the DA-led coalition government in NMB.
At the time, the PA's Gayton McKenzie said they were done with the DA, and would be working with the opposition to remove them.
However, dissent among the opposition ranks spread as the EFF refused to back the PA councillor Marlon Daniels as their preferred mayoral candidate, despite the majority voting in his favour.
After a late-night meeting between senior leaders of the respective parties, McKenzie announced that the PA would not support the vote of no confidence against Trollip, meaning the opposition would not have 61 votes needed to pass the motion.
In November last year, Lawack and Trollip survived motions of no confidence in them.
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