Gerrie Nel, the state attorney, last month appealed to the trial judge over the "shockingly too lenient" six-year sentence she handed down for the murder of Reeva Steenkamp.

Judge Thokozile Masipa rejected the application, saying she did not believe another court "would find differently". But the National Prosecuting Authority said on Friday it would now apply to the Supreme Court of Appeal for a stronger sentence.

Sources in the Pistorius legal team and among his friends said Mr Nel, who was nicknamed The Pitbull during the long-running trial for his ferocious interrogation technique, had "lost his sense of perspective" over the case.

"Gerrie Nel will never let this matter rest, he has a point to prove and he is just going to go on and on until he feels he makes it," a source close to the athlete said.

"He has lost his sense of perspective, he is no longer acting in a dispassionate way, it is clearly a personal vendetta against Oscar.

"But we think it could actually backfire against the state now. The Supreme Court of Appeal judges won't do anything with the sentence to make it longer, they could actually make it shorter and Oscar could be out sooner. We feel relaxed about it, justice will take its course."

A source in the athlete's legal team said they had "had enough" of the endless appeals, but believed Mr Nel was banking on further support from Justice Eric Leach, the chair of the five-man panel at the SCA which replaced 29-year-old Pistorius' original manslaughter conviction with one of murder last year.

"It is a vendetta, that's all it is," the source said. "It's rubbish to say this case sets a precedent, every case is judged on its own merit. They've clearly got a lot of hope that Leach will back them again."

Mr Nel had argued that Judge Masipa had shown too much "maudlin sympathy" for the double-amputee's disability and anxiety disorder when she waived the 15-year minimum convicted murderers are meant to serve in prison under the law, and it sent the wrong message to other would-be killers.

She had pointed to his disability, his "evident" remorse and his agony over the death, which he caused after firing four shots through a locked toilet door at his Pretoria home on St Valentine's Day 2013. He told police he believed his 29-year-old model girlfriend was an intruder.

"He is a fallen hero, he has lost his career and is ruined financially. Having taken a human life in the way he did, he cannot be at peace," Mrs Masipa said.