Oscar Pistorius has filed an appeal to South Africa's Constitutional Court challenging his lengthened prison sentence.

South Africa's Supreme Court of Appeal more than doubled the former Paralympian's prison sentence on 24 November for the murder of his girlfriend Reeva Steenkamp.

His prison sentence was increased from six to 13 years and five months. The court handed him the minimum 15-year murder sentence, subtracting the time he has already served.

Pistorius believes that the Supreme Court's decision violates his rights to a fair trial.

The convicted athlete killed Steenkamp by firing four bullets from a handgun through a closed bathroom door on Valentine's Day 2014. He claims that he thought Steenkamp was an intruder.

He was initially sentenced to six years for culpable homicide in 2014, a prison term that prosecutors described as "shockingly lenient". He only served 10 months of the original sentence before he was released and put under house arrest.

After the sentence was increased, the Steenkamp family spokesperson said: "This is an emotional thing for them. They just feel that their trust in the justice system has been confirmed this morning."

South African legal commentator Llewellyn Curlewis told Sky News: "As far as I'm concerned, this was the correct outcome at long last."

On Monday (19 December), Pistorius filed papers at the Constitutional Court to challenge his increased sentence, News24 reported.

His application stated that the Supreme Court had "impermissibly disregarded or ignored" findings that were considered substantial evidence during the original trial.

"[Pistorius] contends that this is impermissible, having regard to the role of an appellate court in dealing with issues of sentences, and is in breach of the applicant's Constitutional right to a fair trial, which includes the sentencing phase, and is in breach of his Constitutional right to not be deprived of freedom arbitrarily or without just cause," his appeal application reads.