RIO DE JANEIRO (AFP) - Luiz Inacio Lula da Silva may be incarcerated and potentially prevented from standing in October's presidential election but the leftist leader's advantage in polls continues to grow.

Figures published by pollsters Datafolha on Wednesday showed the former president's share of the vote has risen to 39 per cent from 30 last month.

It means he now has more than double the number of his nearest challenger, right-wing candidate Jair Bolsonaro, who is up to 19 per cent from 17 last month.

None of the other 11 registered candidates polled double figures in the sample of almost 8,500 people asked who they intended to vote for in the Oct 7 election.

The figures were in line with other polling organisations that gave Lula a 37 per cent share earlier this week.

The 72-year-old leader of the Workers' Party (PT) is aiming for a third term in the top job having led Brazil from 2003 to 2010.

But Lula could be barred from running as he serves a 12-year sentence for corruption.

Last week, Brazil's Attorney-General Raquel Dodge asked the country's top court for electoral matters to invalidate Lula's candidacy.

But the United Nations Human Rights Committee ruled that he cannot be disqualified "until his appeals before the courts have been completed in fair judicial proceedings".

Brazil is technically obliged to abide by the committee's findings.

In Brazil's presidential election system, the top two candidates from the first round head to a second round run-off, with Datafolha putting Lula ahead of Bolsonaro by 52 to 32 in that vote.