LONDON — South African prosecutors won the right on Wednesday to appeal the acquittal on murder charges of Oscar Pistorius, the double-amputee track star convicted of a lesser offense in the killing of his girlfriend in 2013.
The reversal came in a complex ruling by Judge Thokozile Matilda Masipa who presided over Mr. Pistorius’s trial and, in September, found him guilty of culpable homicide, equivalent to manslaughter. She later sentenced him to a five-year jail term.
The prosecution called the sentence “shockingly inappropriate” to the crime, since Mr. Pistorius could be released on house arrest after 10 months in the hospital wing of a prison in Pretoria, the South African capital.
While Judge Masipa on Wednesday rejected an appeal against the sentence, her ruling meant that prosecutors may now ask the highest appellate court to consider a murder charge that could carry a minimum 15-year term.
The judge’s ruling on Wednesday hinged on a distinction in South African law between culpable homicide — technically a crime of negligence — and a form of murder when a defendant is accused of knowing that his or her actions may cause death.
Mr. Pistorius, 28, has acknowledged killing his girlfriend, Reeva Steenkamp, 29, in the early hours of Feb. 14 last year, when he fired four rounds from a handgun through a locked toilet cubicle door. Throughout his trial, he repeatedly denied knowing that she was on the other side of the door and said that he fired in the belief that an intruder had entered his home.
The prosecution, however, accused Mr. Pistorius of premeditated murder, South Africa’s most serious homicide charge, with a minimum 25-year jail term, saying that the couple had argued and that Mr. Pistorius had fired in a jealous rage.
Starting his appeal against both the conviction and the sentence on Tuesday, the state prosecutor, Gerrie Nel, argued that Mr. Pistorius must have understood the likely outcome of opening fire and that he had therefore committed a form of murder.
In a ruling streamed live on South African news websites from the North Gauteng High Court in Pretoria, Judge Masipa said the issue was “not an easy one.”
On the sentence, she said she was “not persuaded that there was any material misdirection” when she ordered a sentence that some legal experts said seemed to be relatively lenient.
But on the grounds for her verdict, she said the Supreme Court of Appeal should consider whether a principle known as dolus eventualis — under which Mr. Pistorius should have been held accountable for the foreseeable consequences of his actions — had been “correctly applied.”
“I cannot say the prospect of success at the Supreme Court is remote,” she said.
Her ruling represented a victory for South African prosecutors who had suffered setbacks in two high-profile trials: the Pistorius case and the acquittal on Monday of Shrien Dewani, a British businessman accused of arranging the murder of his wife on their honeymoon in Cape Town in 2010.
Mr. Dewani returned to Britain from South Africa on Wednesday.
In the Pistorius case, it was not immediately clear when the appeal would be heard.
“Our argument was that he should have been convicted of murder, and then would have been sentenced to a minimum sentence of 15 years,” said Nathi Mncube, the spokesman for the National Prosecuting Authority.
His trial opened in March and was initially set to last three weeks. With frequent delays and adjournments — including weeks of psychiatric tests, which found the athlete fit to stand trial — the case lasted months and is now set to run further.
At the time of the shooting, Mr. Pistorius, who was born without fibula bones and has been a double amputee since infancy, seemed at the peak of his athletic prowess.
Months earlier, he had competed in both the Olympic and the Paralympic Games in London in 2012. Internationally, he was seen as a star who had drawn on wells of perseverance to overcome his disability, while, in South Africa, he had celebrity status alongside his glamorous girlfriend — a golden couple.
The shooting ended it all. Sponsors abandoned him, and he stopped competing. Later, the Paralympic movement banned him from competition for the five years of his sentence.
In court, he appeared as a distraught figure, frequently overcome by his emotions, weeping and retching as Mr. Nel confronted him with grisly images of Ms. Steenkamp’s bloodstained body.
Some members of the runner’s family took issue with the outcome of Wednesday’s hearing, but others said they would not challenge the judge’s ruling. “It should not have gone this far,” Henke Pistorius, Oscar Pistorius’s father, told reporters.
But a family statement said, “We note the finding of the court and abide by the ruling,” The Associated Press reported.
According to its website, the Supreme Court of Appeal, in Bloemfontein, “sits in panels of five or three” members drawn from the court’s 22 judges. “There may be more than one judgment in a case if there is a difference of opinion. The decision of the majority is the decision of the court.”
According to the website, the court is not set to meet again until Feb. 15.
Witnesses do not appear before the Supreme Court of Appeal, which makes its decisions known in written judgments. The court’s rulings are based on the record of proceedings in lower courts, and the parties “need not be present during the hearing of an appeal,” according to the website.
Correction: December 10, 2014
An earlier version of this article misstated the month in which Judge Thokozile Matilda Masipa acquitted Oscar Pistorius of murder and found him guilty of culpable homicide. The verdict was issued in September, not October.