His future with England is up in the air, with England managing director Hugh Morris stating that further talks are needed before he is welcomed back into the fold.
In a statement released by the ECB, Pietersen apologised for sending "provocative texts to my close friends in the SA team" during the second Test.
The South African-born batsman has been dropped for the Lord's Test this week as a result of the controversy despite scoring 149 in one innings of that match.
He said: "The texts were meant as banter between close friends. I need to rein myself in sometimes.
"I truly didn't mean to cause upset or tension, particularly with important games at stake...
"I did send what you might call provocative texts to my close friends in the SA team.
"I apologise to Straussy (England captain Andrew Strauss) and the team for the inappropriate remarks at the press conference and for the texts."
England need to win the third and final Test to preserve their status as the number one Test team ahead of South Africa, who lead the series 1-0.
Morris said further talks would determine whether Pietersen could return.
"We are in receipt of Kevin's apology, but further discussions need to take place to establish whether it is possible to regain the trust and mutual respect required to ensure all parties are able to focus on playing cricket," he said.
"Critically, those discussions should take place behind closed doors, rather than in the media spotlight.
“A successful conclusion to this process is in everyone’s best interests and is required for Kevin Pietersen’s potential selection in all forms of the game to be considered."
Strauss, who was reportedly one of the subjects of the texts, also said the wounds would need time to heal.
"There are ongoing issues with trust and mutual respect which do not get solved overnight," he said. "[But the apology] is a first step, definitely."
"Who knows what's going to happen in the future? It's a bit of a process that we all have to go through."
Strauss added: "I'm not willing to state one way or another whether it's going to be possible or impossible on that matter."
Strauss welcomed Pietersen's admission of guilt and his subsequent apology but has been dismayed the 'trust and mutual respect' within England's dressing room has been disrupted.
"If there is a way forward the issues regarding trust and mutual respect have to be dealt with and it's not just a one-way process and we need to deal with those behind closed doors.
"The England cricket team has been in the news for the wrong reasons and we all want to move forward but there's some issues regarding trust and mutual respect that don't get dealt with overnight.
"My personal point of view is that I prefer those issues to be dealt with in private, away from the media and that's certainly the way I'll be approaching it going forward."
Proteas captain Graeme Smith defended his team, saying: "We've stayed away from the rubbish... and we've just focused on the cricket."
A sticking point for Pietersen had originally been his required participation in the T20 and ODI teams as well as the Test side as part of his central contract.
After the second Test he intimated that he would retire from all international cricket. He came in for stinging criticism for that stance, with many commentators saying he was looking to use his temporary position of power - he won the man-of-the-match award - to achieve his own ends.
Selectors responded by dropping him for the third Test, despite his subsequent claim in a video uploaded to YouTube that he was still committed to England.
The ECB was also angry that he failed to confirm publicly last weekend that the texts reportedly sent about his team-mates did not exist, or if they had, that he had failed to apologise for them.
Pietersen is now said to be willing to play for England in all matches despite the commitments taking him away from part of the lucrative T20 Indian Premier League.