Brazil's top electoral court said it would make a decision Friday on jailed former president Luiz Inacio Lula da Silva's eligibility to run in October's presidential elections, a source told AFP.
The left-wing icon is leading polls but has been incarcerated since April for accepting a bribe, and could be barred from standing for a potential third term in office under Brazil's clean slate law.
Lula's case was a last-minute addition to the Superior Electoral Court's extraordinary session, due to begin at 2:30pm (1730 GMT) in Brasilia.
The court will also decide whether or not Lula can appear in television commercials that are due to begin circulating on Friday.
Lula, 72, has been sentenced to 12 years in prison for accepting a luxury seaside apartment as a bribe from a construction firm.
He vehemently denies the accusations and has dismissed the charges as a political plot aimed at preventing him from standing in the elections.
Lula has received backing from the UN Human Rights Committee, which two weeks ago ruled that he cannot be disqualified from the elections as his legal appeals are ongoing.
Committee member Olivier de Frouville told AFP that Lula should be allowed to "organize and campaign, even from jail."
Brazil is technically obliged to abide by those findings, but the government in Brasilia described the committee's conclusions as "a recommendation" that is "not... legally binding."
Despite the uncertainty over his ability to stand, Lula currently leads polls with more than double the share of his nearest challenger, the right-wing Jair Bolsonaro.
Lula retains great popularity in Brazil after a successful spell in the presidency from 2003-10, but his Workers' Party's plan B looks hopeless as former Sao Paulo governor Fernando Haddad, who will stand if Lula is barred, commands little popular support.
However, the political chaos has had wider repercussions in the country with the currency losing 20 percent of its value against the dollar since January.
Lula's social media followers remain upbeat, though, and the Workers' Party launched an appeal for support on Twitter, after which a hashtag translating to "Lula on the ballot box" quickly began trending.