A majority of Brazil's Federal Supreme Court found former president Jair Bolsonaro guilty on Thursday of leading a conspiracy to remain in power after his 2022 election defeat, in a ruling that could send the country’s most prominent right-wing leader to prison for decades.

Three of the court's five justices voted to convict Bolsonaro of plotting a military coup and of participation in an armed criminal organisation that sought to overturn the result of the 2022 presidential election. One justice voted to acquit and a final justice had not cast a vote at the time of the ruling; the simple majority was sufficient to register the conviction. Bolsonaro is due to be sentenced on Friday.

According to reporting from the court session, Justices Carmen Lúcia, Alexandre de Moraes and Flávio Dino voted to convict. Justice Luiz Fux was the only justice to vote to acquit in the session that produced the majority. The court found that the plot failed to secure the necessary backing within the armed forces but nevertheless culminated in the storming of government buildings by Bolsonaro supporters on Jan. 8, 2023.

Prosecutors and several justices described the actions surrounding the post-election period as an organised effort to keep Bolsonaro in office despite his loss to Luiz Inácio Lula da Silva. The January 2023 attack on the presidential palace, Congress and the Supreme Court in Brasilia was a central episode in the trial, and the ruling ties the former president to the events and to attempts to enlist military support for an unconstitutional retention of power.

Bolsonaro, a former army captain who led Brazil from 2019 to 2022, has long expressed admiration for the 1964-1985 military regime. His rise from a backbench congressman to the presidency was marked by a combative political style and the cultivation of a powerful conservative base. He narrowly lost the 2022 election to Lula and repeatedly cast doubt on the result in the months that followed, according to court findings.

Legal experts said the conviction marks an unprecedented outcome in modern Brazilian politics: no former president had previously been convicted for plotting against the democratic order. The ruling adds to a broader pattern of legal scrutiny of populist leaders worldwide in recent years.

Bolsonaro faces multiple criminal investigations related to his conduct during and after his presidency. Sentencing on the conviction delivered Thursday will determine the length of any prison term; commentators have said the penalties could amount to decades behind bars if multiple counts and penalties are aggregated.

Allies of Bolsonaro have denounced the proceedings. Media reports said former U.S. President Donald Trump has criticised the trial, calling it a "witch hunt," and Bolsonaro supporters in Brazil have vowed to contest the legal process through appeals and political mobilisation. The Supreme Court's decision, however, is a major legal milestone and is likely to shape both the immediate political landscape in Brazil and longer-term debates about accountability for attempts to undermine democratic transitions.

The court's majority decision follows months of testimony, documentary evidence and legal argumentation about contacts between Bolsonaro allies and elements of the security forces, as well as the organisation and financing of rallies that preceded the Jan. 8 assault on public buildings. Appeals and further legal motions are expected, and the remaining justice's vote, while not affecting the current conviction, will be recorded in the court's formal ruling.

The case and forthcoming sentence are likely to prompt additional legal filings and could influence political alignments ahead of future elections. For now, the conviction stands as a historic judgment against a former Brazilian head of state accused of attempting to subvert a democratic transfer of power.