Polls show Mr Bolsonaro is on course to take 59 per cent of the vote, leaving his opponent, leftwing academic and former mayor of Sao Paolo, Fernando Haddad, with just 41 per cent.
The Ibope poll, published by the Estado de S.Paulo newspaper on Monday, interviewed 2,506 people and also showed Mr Haddad now has a higher rejection rate, with 47 per cent of those polled saying they would never vote for him, compared with 35 per cent rejecting Mr Bolsonaro.
Mr Bolsonaro, a 63-year-old congressman, has led a populist campaign which has seen him openly defend Brazil's 1964-1985 military dictatorship, call for his political opponents to be killed, promise to relax gun-laws, and attack feminism.
But his pitch as the anti-establishment candidate has won over voters frustrated with political corruption and violent crime.
Meanwhile, Mr Haddad, of the Workers Party (PT), has so far been unable to distance himself from the disdain many Brazilians harbour for his party and its imprisoned founder, former President Luiz Inacio Lula da Silva, who was convicted on corruption charges last year.
The PT held the presidency for 13 of the last 15 years and Lula remains loved by swathes of the population for his social policies, credited with easing the lives of the poor in one of the world's most unequal countries.
But the stunning levels of political graft that flourished under the PT governments, revealed by federal investigators in the last four years, have effectively kneecapped Mr Haddad's run.
He has struggled to both stick by Lula, whom the PT considers an unjustly convicted political prisoner, and also acknowledge the party's errors, promising to clean up the loathed political system.
Pollsters had previously said they expected Mr Bolsonaro’s popularity to reach a limit, particularly given his repeated attacks on black people, the LGBT+ community, women and feminism.
Over the course of his career he has said he would not pay women the same as men, told a fellow congresswoman she “didn’t deserve to be raped”, called the secretary of women’s policy a “big dyke”, and said he is “in favour of dictatorship”.
During his campaign his son Eduardo, who is also a congressman, said: “Rightwing women are prettier than leftwing women. They don’t show the breasts in the streets, nor do they defecate in the streets. Rightwing women are more hygienic.”
But his campaign has confounded expectations, growing greater momentum and seeing more and more supporters flock to back him, with his popularity among women growing.
Mr Bolsonaro remains unable to lead street rallies as he recovers from a near-fatal stabbing last month.
The run-off vote between the two candidates takes place on 28 October.
Agencies contributed to this report
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