A psychiatrist assessing Oscar Pistorius has been struck down by a heart attack, only a day before signing off his final report on the fallen paralympian.

It is not yet known whether the event will cause the trial to be delayed, but another postponement is believed to be unlikely, according to the disgraced runner's defence team.

Dr Leon Fine suffered a heart attack last night (Thursday) after spending a month as part of a team of four mental health experts evaluating Pistorius at Weskoppies psychiatric hospital, in Pretoria.

Fine's report about whether the runner has a mental health problem was due to be submitted on Friday, said South African media outlet, Enca. But it has not yet been signed.

Judge Thokozile Masipa is set to use the reports to rule on whether the runner was responsible for his actions, when he shot dead girlfriend Reeva Steenkamp.

Pistorius denies premeditated murder and claims he mistook Steenkamp for an intruder inside his luxury home on the outskirts of Pretoria. The prosecution alleges he killed her following an argument.

Pistorius also faces charges of illegally firing a gun in public and of possessing ammunition, both of which he denies.