A second round in the October presidential election in Brazil becomes more likely as the opposition gains ground, according to a poll released on Wednesday.

The Ibope poll surveyed 2,002 eligible voters in 143 towns throughout Brazil on July 18-21.

It shows President Dilma Rousseff, who is running for re-election, has 38 percent of voter support, down from 39 percent in a June poll.

Her main rival Senator Aecio Neves, of the Brazilian Social Democratic Party (PSDB), has 22 percent support, up from 21 percent in June.

Third-placed candidate Eduardo Campos, of the Brazilian Socialist Party (PSB), has 8 percent, down from 10 percent in the previous poll.

The other eight candidates together account for 8 percent of the votes, while some 16 percent of those surveyed said they plan to vote blank and 9 percent said they were undecided.

It remains unclear whether there will be a runoff, but the poll points to its likelihood.

In a runoff, the poll shows, Rousseff would get 41 percent of the votes, against Neves' 33 percent or Campos' 29 percent.

While Rousseff has the biggest voter support, she also has the highest disapproval rate, 36 percent. In contrast, Neves has 16 percent and Campos 8 percent.