Charles' statement will say he was "deceived" by Ball and that he was unaware of the true nature of the clergyman's behaviour until more than 20 years after allegations first surfaced, it was reported last week.
The statement is due to be read on July 27, as the inquiry examines how abuse allegations against the former Church of England bishop were handled.
Ball, jailed in 2015 for sexually abusing 18 young men over 30 years, previously boasted of his links to royalty and was said to be a confidant of Charles.
At the outset of a five-day case study into Ball, IICSA chairwoman Professor Alexis Jay said the leak was a "very serious breach of confidence by someone with direct access to information in this investigation" and promised "firm action" should the source be uncovered.
She said the inquiry panel was "disappointed" at the breach, describing it as something that "not only undermines the authority of the inquiry to get to the truth but can also erode public confidence in the inquiry at all levels".
All core participants and those with access to the statement will be asked to confirm in writing whether they shared it with any unauthorised party, she added.
"I have already set in motion an investigation to identify the source of this leak and will take firm action to protect the integrity of the Inquiry should the source be identified," said Prof Jay, who said the statement had been reported by both the Times newspaper and the BBC.
Counsel to the inquiry Fiona Scolding QC said a suggestion a source had made to the Times that the timing of the statement was deliberate to prevent Charles being called for cross examination "could not be further from the truth".
She said: "There is no requirement for him to attend in person as his evidence, whilst important, is not of central relevance to many of the issues raised by this case study."
The inquiry is examining how the Church of England handled allegations of sexual abuse and has previously focused on the Diocese of Chichester, where Ball and several other convicted paedophile priests once officiated.
Ball accepted a caution for one count of gross indecency in 1993 and resigned due to ill health.
Now 86, the former bishop of Lewes and then Gloucester, was released in February last year after serving half his 2015 sentence behind bars.
He is too unwell to give evidence either at the inquiry or by videolink, Ms Scolding said, but two statements will be submitted.
In one he makes an apology and identifies that he has "neither
been open nor shown penitence in the past", the inquiry heard.
Opening the hearing in central London on Monday Ms Scolding said of Ball: "His fall from grace was a huge shock to the church.
"He was a very senior clergyman with enormous spiritual authority. He also had power and charm."
She said that power had been "further enhanced by his cultivation of influential friends both within and outside the church".
"He did not stint from mentioning those friends in prominent places when he thought it would assist his cause," she added.
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