BRASILIA, May 14 (Xinhua) -- Right-wing candidate Jair Bolsonaro leads the polls before Brazilian presidential elections in October as former president Luiz Inacio Lula da Silva has been jailed for corruption.

According to a new poll published by the MDA Institute on Monday, Lula would still be the broad favourite but he was jailed this year for 12 years on corruption charges. It's likely that he will be declared ineligible to run by the Superior Electoral Court.

Without Lula in the race, Bolsonaro has 19.7 percent of the vote, Marina Silva, a former environmental minister, stands at 15.1 percent, and Ciro Gomes, of the left-wing Democratic Labor Party (PDT), has 11.1 percent.

The former governor of Sao Paulo, Geraldo Alckmin, of the Brazilian Social Democracy Party (PSDB), has 3.8 percent, and the Workers' Party Fernando Haddad is on a lowly 3.8 percent.

In two other scenarios, with a broader field of candidates, Bolsonaro remained ahead with 20.7 percent and 18.3 percent respectively.

Bolsonaro may struggle to conquer new voters in a second round run-off, with 52.8 percent of voters saying they would never vote for him.

However, the real concern for the country may come from over 50 percent of the electorate either being undecided or set to cast a protest vote.

Some 27.4 percent of respondents said they would spoil their ballot or leave it blank while 39.6 percent said they were undecided who to vote for.

The first round of the election will be held on Oct. 7, with the top two candidates then facing off in the second round on Oct. 28.