Brazil's Congress has overturned a veto of a bill that would dramatically reduce former president Jair Bolsonaro's prison sentence for plotting a coup after he lost the last election.

Last year Bolsonaro was sentenced to 27 years in jail for his attempt to cling to power after losing the 2022 elections to Luiz Inacio Lula da Silva.

Lula had tried to block a subsequent push by the conservative-majority Congress to reduce Bolsonaro's term to just over two years, but in a tense session on Thursday lawmakers overrode his veto of a law changing how prison sentences are calculated.

The law could still be challenged in the Supreme Court.

Bolsonaro, 71, was in March allowed to temporarily be placed under house arrest due to ill health.

The former army captain, who served one term as president from 2019 to 2022, had known of plans to assassinate Lula and his vice-presidential running mate, Geraldo Alckmin, Supreme Court justices said as he was found guilty in September.

Without the backing of the senior leaders of the country's armed forces, the conspiracy failed and Lula was sworn in without incident on 1 January 2023.

After Lula's veto was defeated, several lawmakers loudly chanted freedom and Flavio Bolsonaro - the name of the imprisoned ex-leader's eldest son who has aspirations for the office once held by his father.

"I thank the deputies and senators for this very special birthday present," Flavio Bolsonaro, a Brazilian senator, posted on X regarding the veto override that coincided with his 45th birthday.

More than two thirds of Congress voted to pass the legislation which proposes reducing sentences for individuals convicted of coup-related acts.

It's the second blow to the left-wing government from Brazilian lawmakers this week and for Lula, who is campaigning for a fourth term.

The day before the veto was overturned, the Senate rejected Lula's candidate for a Supreme Court justice, Jorge Messias.

It was the first time in decades that the president's picks for a seat on the top court has been rejected, and it marked momentum for the younger Bolsonaro, who is tied with Lula in the polls.