Former Brazilian president Jair Bolsonaro has lost his appetite and continues to vomit, his son says, just days before learning his fate in a trial for plotting an alleged coup.

"The old man is thin, has no desire to eat and continues facing endless bouts of hiccups and vomiting," Carlos Bolsonaro, one of the former president's sons, said on X.

"It hurts too much to see all this."

The 70-year-old, who ruled Brazil from 2019 to 2022, is on trial along with seven co-accused for allegedly plotting to illegally cling onto power after losing elections to leftist Luiz Inacio Lula da Silva.

The Supreme Court is expected to begin handing down its verdict in the case on Tuesday.

The former army captain, who has the backing of US President Donald Trump, faces 40 years in prison if convicted.

Bolsonaro has been under house arrest in the Brazilian capital for almost a month and must wear an electronic ankle monitor, restrictions imposed as part of an investigation into alleged attempts to disrupt the trial against him.

The former head of state suffered similar symptoms in July – hiccups and vomiting linked to digestive problems stemming from a stab wound he received during a 2018 campaign event.

The health issues led to medical tests in August that revealed he had recently suffered two bouts of pneumonia.

In April, Bolsonaro underwent surgery to repair an intestinal obstruction, one of several medical interventions required since the 2018 stabbing. He remained hospitalized for three weeks after that operation. (AFP)