The National Prosecuting Authority's (NPA's) decision to drop charges against African National Congress (ANC) president Jacob Zuma, was a tipping point that could lead to the erosion of the rule of law, a senior advocate has warned, The Star reported on Thursday.

"We may later see this as a tipping point leading to a slippery slope to the erosion of the rule of law," constitutional law expert Wim Trengove SC told a seminar at the University of Cape Town on Wednesday.

According to Trengove, acting NPA chief Mokotedi Mpshe's decision was based largely on peripheral considerations, such as "eleventh hour shenanigans relating to the timing of the prosecution".

Trengove represented the NPA several times in opposing Zuma's legal bids to halt his prosecution.

He said Mpshe had not sufficiently weighed up public interest in seeing justice done, particularly regarding those alleged to have abused public office.

Mpshe had failed to apply a two-step test he had set himself. In addition to determining whether the case had been "fatally infected" by manipulation, he should also have determined whether this contamination outweighed the public interest in seeing justice done.

Mpshe's conduct "seems to be that of one only too relieved to find an escape from unbearable pressure", The Star quoted Trengove as saying.