BRASILIA, Aug 29 (Reuters) - Environmentalist Marina Silva has widened her lead over President Dilma Rousseff to 10 percentage points in a likely second-round runoff in Brazil's October election, a survey by polling firm Datafolha showed on Friday.
It was the third poll in less than a week that projected the Oct. 5 election would go to a second-round vote in which Rousseff would be unseated by Silva, who entered the race just 10 days ago following the death of her party's candidate.
Silva has surged to 34 percent from 21 percent in a previous Datafolha poll conducted before she launched her candidacy, while Rousseff's support has fallen to 34 percent from 36 percent in a first-round vote.
If the race goes to a runoff on Oct. 26 as expected, Silva would win 50 percent of the votes, while Rousseff would get 40 percent, widening her margin of victory from six to 10 percentage points, according to Datafolha.
Support for centrist candidate and market favorite Aecio Neves has dropped to 15 percent from 20 percent in the previous poll on Aug. 18.
The poll of 2,844 eligible voters was conducted Aug. 28-29 and has a margin of error of plus or minus two percentage points. The results were broadcast on TV Globo's nightly news program. (Reporting by Anthony Boadle and Alonso Soto; Editing by Ken Wills)