President Luiz Inacio Lula da Silva's chief of staff quit on Thursday over an ethics scandal, in a bid to stave off possible damage to Lula's front-running candidate in Brazil's presidential election next month.

Erenice Guerra, whose role as cabinet chief is an influential post in the Lula government, had come under growing pressure to step down over allegations that she was involved in a kickback scheme for public works contracts.

Trailing badly in opinion polls, main opposition candidate Jose Serra has sought to link the scandal to ruling party contender Dilma Rousseff, who preceded Guerra as Lula's chief of staff before launching her campaign.

Rousseff holds a wide lead in polls ahead of the Oct 3 election and remains on course to win a majority of votes in the first round of voting, despite the allegations that she condoned unethical conduct by her leftist Workers' Party.

Guerra has been accused of influence peddling and helping her son's consultancy skim money from public contracts, including while she was Rousseff's advisor before becoming chief of staff.

But Rousseff has not been directly linked to any wrongdoing, which analysts say makes it unlikely the scandals will cut her lead.

"The chances that it forces a second round are still rather small. The opposition would have to be very skilled to exploit this incident," said Amaury de Souza, a Rio de Janeiro-based political analyst.

"The corruption scandals so far didn't change voter preference except among those with higher education or income. That is insufficient to change the odds."

Guerra's resignation could even boost Rousseff's support if it is seen by voters as a decisive move to purge an official involved in corruption, said Jose Luciano Dias, a political analyst with CAC consultancy in Brasilia. "Dilma will even win some votes. The government will say they investigated and took action," he said.