Oscar Pistorius' conviction for killing his girlfriend has been changed from manslaughter to murder.
An appeals judge said last year's verdict was "confusing and flawed" and the athlete will now be re-sentenced by the original court and will almost certainly go back to jail.
A murder conviction carries a much heftier sentence: a minimum of 15 years in South Africa.
Justice Leach said Pistorius must have foreseen that firing into the toilet would likely have killed whoever was inside - but he went ahead anyway.
Reeva Steenkamp's mother, June, closed her eyes but remained silent as the decision was read out in the Bloemfontein court.
The athlete was not in court and is currently under house arrest at his uncle's luxurious home after being released in October.
He served less than a year of a five-year sentence for killing his girlfriend on Valentine's Day 2013.
Pistorius had said he accidentally shot dead Ms Steenkamp after believing there was an intruder in his house. The prosecution argued he killed her after an argument.
The Paralympian was originally convicted of culpable homicide - akin to manslaughter - after a trial under the glare of the world's media.
The appeal centred on dolus eventualis - a Latin legal phrase meaning the act was carried out knowing it would lead to death and the accused went ahead anyway.
The appeal court said the concept had been incorrectly applied by Judge Masipa in the original trial.
It also said that circumstantial evidence - such as Whatsapp messages and certain ballistics evidence - was not properly considered.
She must now give Pistorius a new sentence, likely to be in the new year.
The appeals court verdict described the case as a "human tragedy of Shakespearean proportions".
Reporter Alex Crawford, at the court, said that could turn into a "mini trial" with Pistorius' lawyers allowed to plead mitigating circumstances.
"That he feels a lot of remorse, that he's served almost one year already - using all that to try to bring down the sentence as low as possible," said Crawford.
"But one thing is for certain, Pistorius faces another longer spell in jail".
Reacting after the decision, Reeva Steempkamp's father told reporters he was "satisfied with everything now".
Barry Steenkamp said: "I would hope to God that all of this could have been prevented, but seeing that it has been done, let us now all get on with our lives".
Listen to an excerpt from the judge's ruling here:
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