Ex-England batsman Pietersen stated in his autobiography that some members of the team were given the password to the account, which was set up by cricket fan Richard Bailey - who is a friend of Stuart Broad.
The 34-year-old's fall-out with the ECB has been well documented since his sacking in February, and the release of his book this week has served to ignite media interest further.
Former England captain Stewart revealed on Wednesday that he had passed on information given to him by Bailey to the ECB.
"I said to him some of it [the account] was very funny and that he had got some good information," Stewart told The Telegraph.
"He said 'yes I do'. He then said, 'Can you keep a secret?' I said, 'it depends'. He went away and then came back and named three players who had access to the account password.
"If that was the case it did not sit well with me. I passed it on to the ECB and it was up to them how they dealt with it.
"I was doing it for the good of the England team and how the hierarchy dealt with it was up to them but I felt they had to be made aware of it."
Stewart confirmed Broad, Graeme Swann and Tim Bresnan were the players named by Bailey.
"I absolutely stand by what is written in Pietersen's book," he said. "I went and told [former ECB managing director] Hugh Morris and [former England coach] Andy Flower on separate occasions what I had been told by this fella [Bailey]."
Broad released a statement at the time saying he had no involvement in the account, while Swann said likewise in his column for The Sun on Tuesday.