A lawyer for Oscar Pistorius has moved to quash reports the athlete "lost it" upon learning his sentence can be appealed by prosecutors keen to prolong his prison stay.

Last week, it was announced that prosecutors had won the right to appeal the culpable homicide verdict given to Pistorius for killing girlfriend Reeva Steenkamp. The appeal could increase Pistorius' sentence to 15 years and bring him yet more crippling legal bills.

A fellow inmate at the Kgosi Mampuru II penitentiary in Pretoria, where Pistorius is serving his time, claimed he took the news badly by "stomping off" in a temper to work out in the gym "like it was going out of fashion".

However, according to lawyer Brian Webber, the account claiming Pistorius stormed off for an impromptu gym session was "sensationalist".

Webber also denied Pistorius heard it on the radio and insisted it was he who imparted the news, as a member of his legal team.

"It is not only sensationalist but also devoid of any truth," Webber told AFP. "I personally went to see Oscar straight after Judge [Thokozile] Masipa gave her ruling and discussed it at length with Oscar.

"This was the first time Oscar received any information on what had transpired in court."