Oscar Pistorius’s lawyer has denied that the jailed athlete had “thrown a wobbly” after learning he could face a much harsher punishment for killing his girlfriend.

Prosecutors on Wednesday won the right to appeal for Pistorius to be convicted of murder instead of the lesser charge of culpable homicide, for which he was sentenced to five years’ imprisonment.

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If the appeal is successful the Paralympian, known as the “Blade Runner” for his prosthetic legs, could face at least 15 years in jail.

South Africa’s Times newspaper reported on Thursday, under the headline “Blade Runner throws a wobbly in jail”, that Pistorius had “lost his cool” after hearing that his acquittal on the murder charge would be appealed.

It quoted a fellow prisoner as saying that Pistorius headed straight to the gym and “started lifting weights like it was going out of fashion” when he heard the news on the radio.

Lawyer Brian Webber said the story “is not only sensationalist but also devoid of any truth”.

“I personally went to see Oscar straight after Judge (Thokozile) Masipa gave her ruling and discussed it at length with Oscar.”

Pistorius, 28, said he shot his model girlfriend Reeva Steenkamp four times through a locked toilet door in the early hours of Valentine’s Day in 2013, believing she was an intruder.

Prosecutors argued he deliberately killed the 29-year-old law graduate after a row, but this was not accepted by the trial judge.

The case will now go the Supreme Court of Appeal.

Pistorius, who celebrated his 28th birthday last month, has been serving his sentence in a central Pretoria prison since October.

Due to his physical disability, he is being held in the hospital ward of the prison, shielding him from the often brutal overcrowded cells, known for gang violence.