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(Corrects first paragraph to reflect that election is this month)

SAO PAULO (Reuters) - Brazil’s far-right presidential candidate Jair Bolsonaro has a 10-point lead over leftist Workers Party candidate Fernando Haddad, and would tie in a second-round runoff against him this month, an opinion poll showed on Monday.

Bolsonaro, who is recovering from a near-fatal stabbing, has increased his lead ahead of the election this Sunday with 31 percent of voter support in a simulated first-round ballot, the survey by polling firm Ibope said. Haddad has 21 percent.

In the likely case of a runoff vote on Oct. 28, required if no candidate takes a majority, Bolsonaro and Haddad both had 42 percent, with the remaining voters undecided or saying they would annul their ballots.

Haddad surged after he replaced jailed former President Luiz Inacio Lula da Silva on the Workers Party ticket, who was banned last month by an electoral court from running from prison, but his support remained stagnant compared to the same Ibope poll last week.

Bolsonaro gained on Haddad in the runoff scenario. Last week Haddad had 42 percent against Bolsonaro’s 38 percent.

Ibope said support for center-left candidate Ciro Gomes remained stable at 11 percent, while business-friendly former Sao Paulo governor Geraldo Alckmin dropped to 8 percent.

The poll, published by the Estado de S.Paulo newspaper and TV Globo, surveyed 3,010 voters across Brazil on Sept. 29 and 30. It has a margin of error of two percentage points.