JOHANNESBURG (AFP) - Former Paralympic champion Oscar Pistorius faced a return to jail after losing a final bid to appeal his murder conviction for shooting girlfriend Reeva Steenkamp.

His lawyers had applied for leave to appeal to the Constitutional Court, the highest court in South Africa, arguing that the Supreme Court of Appeal (SCA) had wrongly upgraded his conviction to murder.

But the National Prosecuting Authority (NPA), which opposed the application, said the appeal bid had failed.

"The court dismissed the application for leave to appeal because there are no prospects of success," NPA spokesman Luvuyo Mfaku told AFP.

Mr Mfaku said the court's decision had been made on Wednesday.

The former Paralympic champion, 29, will attend a sentencing hearing in Pretoria on April 18.

The minimum 15-year jail term for murder may be reduced due to time he has already spent in prison.

The double-amputee killed Ms Reeva Steenkamp, a model and law graduate, on Valentine's Day three years ago, saying he mistook her for an intruder when he shot four times through the door of his bedroom toilet.

He was released from jail in October to live under house arrest at his uncle's property in Pretoria after serving one year of his five-year prison sentence for culpable homicide - the equivalent of manslaughter.

The SCA judges in December described his testimony at his trial in 2014 as "untruthful" and delivered a damning indictment of the original verdict.

They found him guilty of murder, overruling the culpable homicide conviction.

Legal papers filed at the Constitutional Court by Pistorius' lawyers contended that the SCA had "acted unlawfully and unconstitutionally".

They also accused the SCA of making "errors of law" over the principle of "dolus eventualis" - awareness of the likely outcome of an action - which has been at the crux of the long-running case.

But most experts had dismissed his chances of success in the Constitutional Court.

"The court has exercised its power not hear this matter - that means that they considered it to have no merit," Mr Stephen Tuson, criminal and constitutional professor at Wits University in Johannesburg, told AFP.

"The trial court now will take into account his personal circumstances and decide on a suitable sentence."

Pistorius was last seen in public at a bail hearing in December, after which he was fitted with an electronic monitoring tag.

Under his bail conditions, he has been allowed to leave his uncle's house at set times with official permission, but not travel further than 20 km.

Pistorius denied killing Ms Steenkamp in a rage and during his trial, sobbed in the dock as details of his lover's death were examined in excruciating detail.

"I was overcome with fear," he told his trial. "Before I knew it, I'd fired four shots at the door."

He has since lost his glittering sports career, lucrative contracts and status as a global role model for the disabled.

Ms Steenkamp's family was not immediately available to comment.

They previously welcomed his murder conviction and described his Constitutional Court appeal as a "delaying tactic" to keep him out of jail.