South Africa batsman Faf du Plessis has pleaded guilty to the charge of ball-tampering and been fined 50% of his match fee although the team management have called the references to tampering "harsh".

The match referee David Boon said that du Plessis' actions warranted the charge being brought against him, but also said that he was satisfied that it "was not part of a deliberate and/or prolonged attempt to unfairly manipulate the condition of the ball."

South Africa decided not to challenge the charge because of fear over a strong punishment. In their first comment on the controversy Mohammad Moosajee, the team manager, read out a statement: "As a team we proceeded not to contest it...because as per the ICC regulations a full hearing could lead to more severe punitive measure which could include a heftier fine or even a match ban."

South Africa denied claims of ball tampering on the third day when AB de Villiers said "we are not cheats," and continued to do so after the verdict was handed down. "Faf showed no intent to change the conditions of the ball. It is harsh to term it ball tampering," Moosajee read. "It was done inadvertently to dry the ball."

When Graeme Smith was asked if he thought du Plessis' actions tainted South Africa's series-levelling win in any way, his answer was limited to a single word. "No."

An ICC release said: "Before the start of fourth day's play on Saturday, David Boon of the Emirates Elite Panel of ICC Match Referees handed the fine to du Plessis who had pleaded guilty on Friday evening."

Boon said: "I am satisfied that the player's actions warranted the umpires applying clause 42.1.1 of the ICC Test Match Playing Conditions, including the laying of a charge under the ICC Code of Conduct against Mr du Plessis in respect of changing the condition of the ball. After discussions with Mr du Plessis, he has elected not to contest that charge, but I am also satisfied that this was not part of a deliberate and/or prolonged attempt to unfairly manipulate the condition of the ball, and that the imposition of a fine of 50 per cent of his match fee is appropriate considering the circumstances."

The incident occurred two overs after tea on the third day, before the start of the 31st over, following television visuals of du Plessis rubbing the ball near the zipper of his trouser pocket. The TV umpire brought it to the attention of the on-field umpires Ian Gould and Rod Tucker, who called Graeme Smith over for a chat and subsequently changed the ball and awarded a five-run penalty against South Africa, sanctions that are consistent with the penalty for unlawfully changing the condition of the ball.

Du Plessis was charged with an article 2.2.9 offence of the ICC Code of Conduct which relates to "changing the condition of the ball in breach of Law 42.3 of the Laws of Cricket, as modified by ICC Standard Test Match, ODI and Twenty20 International Match Playing Conditions clause 42.1".

Some South African players - JP Duminy after play on the third day and Vernon Philander before play on the fourth - had said they thought there was nothing amiss with the condition of the ball when it was changed.

Penalties for offences such as du Plessis' under Level 2 of the ICC's code of conduct can range from a fine of 50% to 100% of a player's match fee to suspensions for one Test, two ODIs or two T20Is.