Lead prosecutor Nel is a veteran lawyer known for his ability to take on the powerful and politically connected. His biggest scalp is former Police Chief and Interpol Head, Jackie Selebi, who was convicted of corruption in 2010.

During the Pistorius trial, Nel painstakingly crafted a picture of Pistorius as a cold-blooded killer who fired through a door at his cowering girlfriend. For his no-nonsense attitude, Nel earned the nickname 'The Bull Dog' as a prosecutor.

Pistorius says he shot dead Steenkamp by accident after mistaking her for an intruder in his house. "We confirm that advocate Gerrie Nel tendered his resignation on January 30," said the NPA's Spokesperson, Luvuyo Mfaku. He said the resignation "is effective today (Tuesday)" and that Nel said he was resigning to pursue other interests.

Nel was not available to comment.

Pistorius was sentenced to six years in prison in July after being found guilty of murdering his girlfriend Reeva Steenkamp on appeal last December. The track star originally received a five-year sentence for a manslaughter conviction, that was upgraded to murder on appeal.

Nel was challenging Pistorius' six year sentence, arguing it was too lenient as it was less than half the 15-years that state prosecutors sought.

--Reuters--