The Speaker of Brazil’s lower house of Congress, Eduardo Cunha, has announced he is leaving the governing coalition.
The defection, and Mr Cunha’s threat to take his party with him, is a blow to embattled President Dilma Rousseff.
Mr Cunha has accused the government and Prosecutor General Rodrigo Janot of conspiring to incriminate him in a widening corruption scandal at state-run oil firm Petrobras.
Without Mr Cunha’s PMDB party, Ms Rousseff has no majority in Congress.
“I cannot accept that the government uses its machinery to seek the political persecution of those who turn against it,” Mr Cunha said in Brasilia.
His defection followed allegations made this week by a jailed former consultant to Petrobras, Julio Camargo, that Mr Cunha had demanded a $5m (£3.2m) bribe to push through contracts for equipment.
Mr Cunha has denied any involvement, and said Ms Rousseff and Mr Janot were unfairly dragging congressmen into the Petrobras investigations.
His name had appeared earlier this year alongside those of dozens of other congressmen on a list of people whom the police reportedly wanted to interview in relation to the scandal.
The real losers in Brazil’s Petrobras scandal
Thirty-four congressmen and one state vice-governor are under investigation for taking bribes.
Investigators allege firms paid corrupt officials in order to get lucrative Petrobras contracts.
President Dilma Rousseff, who chaired Petrobras when much of the corruption is believed to have taken place, has been cleared of involvement.
Analysts say Mr Cunha’s opposition to Ms Rousseff could become a major obstacle for any legislation proposed by her government, at a time when it is trying to introduce public spending cuts to stave off recession.
Mr Cunha, an evangelical Christian, is seen as a possible presidential candidate in 2018.