Brazil's far-Right election front-runner was accused of setting up a 'criminal network' with big businesses to spread fake news through Whatsapp.

Brazilian media reported that well-heeled supporters of Jair Bolsonaro paid for messaging by third-party agencies, each paying up to 12 million reais ($3.26 million) to spread tens of thousands of attack ads.

Leftist opponent Fernando Haddad, who trails by 18 points, denounced the practice revealed by newspaper Folha de S.Paulo, accusing Mr Bolsonaro of flouting campaign laws by creating "a veritable criminal organisation with businessmen who are using undeclared money to pay for false messages on WhatsApp".

Bolsonaro's campaign did not respond to a request for comment. The candidate's son, Carlos Bolsonaro, accused Folha and Haddad's Workers Party, or PT, of "telling half-truths or decontextualized lies" in a post on Twitter.

Mr Haddad said the PT has witnesses saying Bolsonaro asked business leaders at a dinner in Sao Paulo for funds to pay for the bulk messaging, which he described as undeclared campaign contributions.

Fernando Haddad, Brazil's presidential candidate for the Workers Party, is 18 points behind his rival credit: Andre Penner/AP

PT lawyers asked prosecutors and Brazil federal police to investigate. Illegal under-the-table funding by companies could, if proven, lead to the disqualification of the Bolsonaro ticket, said attorney Guilherme Salles Gonçalves, an electoral law expert.

PT allies said they would file a complaint with electoral courts. Haddad called on international observers and WhatsApp itself to look into the case.

A representative for WhatsApp, a division of Facebook Inc , did not immediately respond to a request for comment.

Mr Haddad has complained frequently that he was the target of false social media information campaigns.

Among other things Mr Haddad, 55, has railed against "libelous" social media posts claiming he tried to have sex education "gay kits" distributed in schools when he was education minister under the now-jailed ex-president Luiz Inacio Lula da Silva.

Bolsonaro, 63, is an ex-paratrooper whose ultraconservative and law-and-order rhetoric has lifted him in the polls.

He has proved himself an adept user of online platforms, largely spurning traditional media outlets and debates in favor of reaching out to millions of followers on his Twitter, Facebook and Instagram accounts.

Bolsonaro easily won the first round of the elections on October 7.