Ex-Prime Minister Gordon Brown says lessons cannot be learned about press standards unless there is honesty about how details of his son's cystic fibrosis were published by the Sun.
He said him and his wife Sarah were "presented with a fait accompli" by the paper, before it ran a story on their son Fraser's medical condition in 2006.
He denied that he or his wife had given permission for the story to be run.
Mr Brown is giving evidence to the Leveson Inquiry into press standards.
Chancellor George Osborne will be giving evidence later and Prime Minister David Cameron will enter the witness box on Thursday.
The inquiry is currently focusing on the relationship between the press and politicians.
The Sun's then-editor Rebekah Brooks had previously told the Leveson Inquiry she had the express permission of the Browns to run the story about Fraser's medical condition, but the Browns have previously said that was "untrue".
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Mr Brown told the inquiry the NHS in Fife had apologised to him because they think it "highly likely" unauthorised information was disclosed by NHS staff about Fraser Brown.
He again denied consent had been given to the Sun to publish the story.
"I find it sad that even now, in 2012, members of the News International staff are coming to this inquiry and maintaining this fiction that a story that could only have been achieved or obtained through medical information or through me or my wife... was obtained in another way.
"We can't learn the lesson about what has happened with the media anything unless there is some honesty about what actually happened, whether payment was made and whether this is a practice which could continue."
He said through the Press Complaints Commission he had attempted to get newspaper editors to agree on limits of coverage about his children.