England all-rounder Ben Stokes has apologised for his "unprofessional reaction" after an altercation with a member of the crowd during the fourth Test against South Africa.
The 28-year-old used expletives towards someone as he made his way off the field after being dismissed for two.
"I wish to apologise for my language that was heard on the live broadcast today after my dismissal," Stokes said.
"I should not have reacted in that way."
Stokes edged a drive to Rassie van der Dussen at first slip off Anrich Nortje to leave England 157-4 in Johannesburg.
England, who lead the four-match series 2-1, closed the opening day on 192-4.
In a statement released on Friday evening, Stokes said he was "subjected to repeated abuse from the crowd" as he made his way towards the tunnel.
"I admit my reaction was unprofessional, and I sincerely apologise for the language I used, especially to the many young fans watching the live telecast around the world," he said.
"Throughout the Tests so far, the support from both sets of fans (England and South Africa) has been brilliant. One incident will not ruin such a competitive series, which we are determined to win."
England director of men's cricket Ashley Giles said Stokes "should not have reacted in the way he did".
But he added several members of staff were subject to "personal abuse" during and after play.
"It is disappointing that a member of the public has gone out of their way to abuse Ben as he was leaving the field," Giles said.
"We have requested to the venue to ensure that security and stewarding are enhanced for the remainder of the match so players and staff members can go about their duties without provocation."
Under the International Cricket Council's code of conduct, the "use of an audible obscenity during an international match" is a level one offence.
A level one offence carries the punishment of a fine of up to 50% of a player's match fee and up to two demerit points.