The decision by South Africa's National Prosecuting Authority could see Pistorius face a murder conviction again for shooting Reeva Steenkamp and, if found guilty on the appeal, a minimum of 15 years in prison.
The 27-year-old double-amputee athlete was acquitted of murder by Judge Thokozile Masipa for shooting Steenkamp multiple times through a toilet cubicle door in his home. He testified he mistook her for a nighttime intruder. Masipa found him guilty instead on a lesser charge comparable to manslaughter.
The NPA said in a statement its decision to appeal was based on a "question of law," meaning it believes that Judge Masipa misapplied the law when she acquitted Pistorius of murder.
"The merits and the demerits of the NPA's argument ... will become evident when we file papers for leave to appeal," the national prosecuting body said. "The prosecutors are now preparing the necessary papers in order to be able to file within the next few days."
Chief prosecutor Gerrie Nel and his assistant, Andrea Johnson, had been "hard at work" since last week studying the judgment, researching and consulting with legal experts, the NPA said.
Prosecutors must apply initially to Masipa for permission to appeal within 14 days of Pistorius' Oct. 21 sentencing. If Masipa grants them permission, Pistorius' case would be reviewed by a panel of three or five judges in the Supreme Court of Appeal, legal expert Marius du Toit said. They could overturn the manslaughter conviction and find Pistorius guilty of murder.
Prosecutors can directly petition the SCA if Masipa denies their appeal, du Toit said.
Pistorius' lawyer, Brian Webber, declined to comment, while Pistorius' uncle, Arnold Pistorius, said in a short family statement: "We take note of the state's announcement. The law must take its course."
Pistorius started serving his prison sentence last week for negligently killing Steenkamp on Feb. 14, 2013. Currently, the multiple Paralympic champion is eligible for release after 10 months to complete the sentence under house arrest.
Prosecutors had said they were disappointed with the culpable homicide conviction and believed they had a strong murder case against Pistorius. A murder conviction has a minimum of 15 years in jail in South Africa.
Chief prosecutor Nel laid the groundwork for the decision to appeal by speaking with Prof. James Grant, a criminal law specialist at Johannesburg's University of the Witwatersrand, prosecution spokesman Nathi Mncube said.
Masipa's decision to acquit Pistorius of murder had been questioned by legal analysts. Grant, a television analyst during Pistorius' trial, was among them and said last week that he had advised Nel to appeal. Grant also said he agreed to assist prosecutors.
Experts say there are grounds for an appeal, partly because the judge may have misapplied a part of the law called "dolus eventualis" — which says someone should be found guilty of murder if they foresaw the possibility of killing someone through their actions and went ahead anyway. The experts questioned how Masipa ruled that Pistorius did not predict that someone might die when he decided to shoot four times from close range into the small toilet cubicle in his Pretoria home, hitting Steenkamp in the hip, arm and head.
Some experts have also said that Pistorius' argument that he did not know it was the 29-year-old model in the cubicle might not be enough to save him from a murder conviction as he still knew there was someone in there when he fired his 9 mm pistol.
Imray reported from Stellenbosch, South Africa.