Oscar Pistorius looks on during his trial at the North Gauteng High Court in PretoriaReuters

The murder trial of Oscar Pistorius is to be delayed while the Olympic and Paralympic athlete undergoes evaluation for mental illness.

The trial is likely to be delayed at least 30 days after judge Thokozile Masipa declared Pistorius should have his mental health assessed.

The request for a mental health assessment came from the prosecution following the testimony of Dr Merryl Vorster, a psychiatrist and expert witness for the defence, who argued Pistorius had an "anxiety disorder," which could have affected his judgment on the night he is accused of murdering his girlfriend Reeva Steenkamp.

The court heard people suffering from a generalised anxiety disorder (GAD) are more likely to fight than flee when faced with a potential threat.

The prosecution wanted Pistorius' mental state evaluated to stop the 27-year-old appealing a potential conviction later on the basis of his mental condition.

Chief prosecutor Gerrie Nel said the claim Pistorius suffers from GAD could be used as a "fall-back" option. He told the court Vorster's testimony came after Pistorius gave his own evidence, in which he was accusing of inconsistency and contradictions.

The defence team opposed the move for an assessment and said the request had "no merit".

"The accused may not have raised the issue that he was not criminally responsible but evidence raised on his behalf clearly raises the issue and cannot be ignored," Judge Masipa told Pretoria's High Court.

Pistorius denies the premeditated murder of Steenkamp and claims he shot her after mistaking her for an intruder inside his Pretoria home on 14 February last year.

The full details of the assessment are due to be read out in court next week. Masipa said Pistorius will be observed as an outpatient at a South African psychiatric hospital.

Pistorius faces life imprisonment if found guilty of murder.