Brazil's top court has suspended Lower House Speaker Eduardo Cunha from his mandate, following a request by the country's attorney general, officials say.
He has been accused of trying to obstruct a corruption investigation against him and intimidating lawmakers.
Mr Cunha is an outspoken critic of President Dilma Rousseff and has led an impeachment drive against her.
In a BBC interview, Ms Rousseff has vowed to "keep fighting".
Next week, Brazil's Senate is due to vote on whether to launch an impeachment trial against her.
If a simple majority votes in favour, she will be suspended from office for 180 days while Vice-President Michel Temer takes over as interim president.
Under Brazil's constitution, Mr Cunha was the next in line for the presidency after Mr Temer, who is facing impeachment proceedings on charges similar to those facing President Rousseff.
Analysis by BBC South America correspondent Wyre Davies
The dramatic suspension of Eduardo Cunha from the lower house of Congress may come too late to save Dilma Rousseff but, be in no doubt, he played a critical role in the process which now sees Brazil's first woman president on the verge of suspension from office.
Dilma Rousseff and her supporters in Congress say it was her government's decision not to give in to the Speaker's demands, specifically over an ethics committee investigation into Mr Cunha, that triggered his subsequent move to begin the impeachment process against the president.
In her wide-ranging BBC interview, while declaring her own innocence, Ms Rousseff was scathing about the man who has emerged as her nemesis.
"The one person responsible for… the impeachment request (against me) is himself under charges. He's notorious, known to have foreign bank accounts and is under charges of money laundering and corruption," she said.
According to opinion polls, Eduardo Cunha is even more unpopular that the beleaguered president after details of his Swiss bank accounts, which he had earlier denied having, came to light.
Mr Cunha, a committed evangelical Christian who often quotes the Bible in his social media messages, protests his innocence and may find a way back.
He is a talented and well-connected political operator but his chances of perhaps becoming the next president of Brazil, by default as Speaker of Congress, are now almost non-existent.
Ms Rousseff has accused Mr Temer and Mr Cunha, who belong to the opposition PMDB party, of being the ringleaders of a "coup attempt" against her.
Mr Cunha is suspected of obstructing an investigation into allegations he took $5m (£3.2m) in bribes from companies seeking to secure contracts with state-oil giant Petrobras, which he denies.
Petrobras is at the centre of a massive kickbacks scandal which has led to the arrest of dozens of lawmakers and top businessmen.
Waldir Maranhao, who is also under investigation for his alleged role in the Petrobras corruption scandal, is to become the next Speaker of the House.