The survey by IPEC showed Lula with 44% of voter support against 31% for Bolsonaro in the first round of the election scheduled for Oct. 2, compared to 44% and 32% respectively in the previous poll.
In the expected runoff, Lula would get elected by 52% of the voters versus 36% for Bolsonaro, a 16-point gap, up from 13 points a week ago, the poll showed.
Bolsonaro has also posted a slight worsening in his approval rate, which dropped to 30% from 31% a week ago, despite efforts after passing welfare programs and measures to tackle inflation.
His disapproval rate was 43%, according to the poll, the same result a week ago.
Pollster IPEC, formerly known as IBOPE, interviewed 2,512 people in person between Sept. 2-4. The poll has a margin of error of 2 percentage points up or down.
(Reporting by Carolina Pulice, Anthony Badle and Pedro Fonseca; Editing by Kim Coghill and Michael Perry)