SAO PAULO, Sept 17 (Reuters) - Brazilian Senator Flavio Bolsonaro and President Luiz Inacio Lula da Silva remain statistically tied in a simulated runoff ahead of next month's election, an AtlasIntel/Bloomberg poll showed on Thursday, with the right-wing challenger holding a narrow lead within the survey's margin of error.
In a runoff, the eldest son of former President Jair Bolsonaro would receive 47.2% of voter support, compared with 46.8% for the leftist incumbent, the poll said.
In a September 10 poll, Bolsonaro led Lula 46.4% to 46.2%.
In a first-round scenario, Lula would receive 44.1% of voter support, followed by Bolsonaro with 41.7%, Renan Santos with 5.1%, and Augusto Cury with 3.5%.
The previous poll showed Lula at 43% and Bolsonaro at 37.4% in the first round.
If no candidate wins more than 50% of valid votes in the October 4 election, the two frontrunners advance to a runoff on October 25.
AtlasIntel surveyed 5,018 people between September 11 and 16. The poll has a margin of error of 1 percentage point.
(Reporting by Gabriel Araujo in Sao Paulo and Michael Susin in Barcelona
Editing by Tomasz Janowski)