SAO PAULO, Aug 11 (Reuters) - Brazil's President Luiz Inacio Lula da Silva's ‌advantage over Senator Flavio Bolsonaro in ‌a potential runoff ahead of October's general election narrowed, ​a CNT/MDA poll showed on Tuesday.

Lula would receive 48.0% of the votes in a runoff, compared with 39.1% for Flavio ‌Bolsonaro, son of ⁠former President Jair Bolsonaro, according to the poll.

In a June survey, ⁠Lula was seen with 49.3% of the votes, and Bolsonaro with 36.8%.

In a ​first-round scenario, ​Lula would lead ​with 42.4% of the ‌votes, virtually unchanged from the June survey, followed by Bolsonaro's 28.7%, Ronaldo Caiado's 4.0%, Romeu Zema's 3.3% and Renan Santos' 2.8%.

If no candidate wins more than 50% ‌of valid votes in ​the first round, the ​two frontrunners ​advance to a runoff.

CNT/MDA interviewed 2,002 ‌people from August 5 ​to 9 ​in 140 Brazilian municipalities. The survey's margin of error is 2.2 percentage ​points in ‌either direction.

(Reporting by Isabel Teles; Writing by ​Fernando Cardoso; Editing by Chizu ​Nomiyama and Keith Weir)