Alastair Campbell, the former Number 10 communications director, has said the "craft" of journalism is almost dead and ordinary people are used as news "commodities".
"I remember watching when Madeleine McCann went missing and I thought about writing to them. I could see the media using them and building them as a news commodity," he told the Leveson Inquiry.
He said her parents Gerry and Kate McCann became "anything goes people". "How nobody from the PCC didn't stand up and say what's going on here amazes me."
On the hounding of Britney Spears when she was clearly "disturbed" he said: "Does nobody say we should stop doing this? I don't think they do."
He added: "There's no transparency about the journalistic practices that they use to fill their papers.
"The public out there are horrified by what they've heard in the last two weeks ... my argument this is not atypical. This is what happens to anybody who they decide is a major news commodity".
There was controversy at the weekend after his evidence was leaked to a blogger and published.
A draft of his written evidence was posted online on Sunday by Paul Staines, who blogs under the name Guido Fawkes. The move prompted Lord Justice Leveson to make an order banning advance publication of documents submitted to the inquiry.
The inquiry will also hear from former Merseyside Police Inspector Alec Owens, who led Operation Motorman - the 2003 Information Commissioner's Officer (ICO) investigation into the media's use of inquiry agents in 2003.
On Tuesday the inquiry heard claims that News of the World (NotW) journalists hacked phones for their former editors Rebekah Brooks and Andy Coulson.
Paul McMullan, who worked for the now-defunct Sunday tabloid for seven years, said editors were aware their staff were illegally accessing voicemail messages but threw them "to the wolves" by denying all knowledge of it.
He described former editor Mrs Brooks as "the criminal-in-chief", alleging she "moulded" David Cameron before he became Prime Minister.
Ms Brooks and Mr Coulson have denied all knowledge of phone hacking.