London (CNN) -- British newspapers print "complete nonsense" in support of agendas set by their owners, former Tony Blair spokesman Alastair Campbell said Wednesday.
"The impact of the story is deemed to be far more important than the accuracy," he said.
Campbell blamed "an obsession with celebrity, a culture of negativity, and amorality among some of the industry's leaders" for driving a "downmarket trend" in the industry.
"Speed now comes ahead of accuracy, impact comes ahead of fairness, and in parts of the press anything goes to get the story first," the former journalist turned spin doctor said.
And self-regulation by the press has "failed," because chairmen of the Press Complaints Commission have been "political fixers operating in the interests of the press, not the public."
He was testifying at the Leveson Inquiry, a wide-ranging British government-backed inquiry into press ethics and practices in the country.
Shortly before he was due to begin testifying, police arrested a 31-year-old woman in connection with their own investigation into phone hacking, they announced.
The woman, who was not named, is the 15th person arrested in connection with the phone-hacking probe, which police are running separately from the Leveson Inquiry. They are also looking into computer hacking and bribery of police by journalists.
The inquiry was prompted by widespread public outrage at the revelation that the Rupert Murdoch-owned tabloid News of the World had hacked into the voice mail of a murdered 13-year-old girl.
Murdoch's son James, the chief executive of the News Corporation subsidiary that published the now-defunct tabloid, has repeatedly denied knowing about the scale of illegal eavesdropping at his papers.
But a former News of the World journalist testified Tuesday that the editors of the tabloid knew that their reporters were hacking phones in search of stories.
Paul McMullan named Andy Coulson, who went on to become an adviser to British Prime Minister David Cameron, and Rebekah Brooks, a Murdoch protege, as editors who were aware of the practice.
Coulson resigned as editor of the tabloid in 2007 when one of his reporters went to prison for hacking the voice mails of Prince William's staff, and he later became Cameron's communications director.
Coulson has always denied knowing about phone hacking, saying he quit the paper because he was ultimately responsible for the actions of his staff.
The Leveson Inquiry has been hearing from high-profile figures for more than a week.
McMullan said phone hacking was widespread at the tabloid and described being caught out himself while trying to hack David Beckham's phone.
The soccer star shocked him by answering the phone before McMullan could dial the code to access his voice mail, forcing the journalist to hang up quickly, he said.
He argued that the public was complicit in the country's tabloid culture, saying newspapers publish stories about stars because "there is a taste for it, there is a market for it."
Celebrities also use the tabloid press to increase their exposure, he said, describing how model Katie Price, better known as Jordan, gave him the finger through a window.
He snapped a picture of her, thinking, "Thanks, love," and sold it for 2,000 pounds ($3,100).
"She knew exactly what she was doing," he said.
He also defended the tabloids' methods as part of a free society, saying he would not want to live in a country where only the secret services were able to hack phones.
Police investigating phone hacking by journalists say that about 5,800 people, including celebrities, crime victims, politicians and members of the royal family, were targets of the practice by journalists in search of stories.
It involves illegally eavesdropping on voice mail by entering a PIN to access messages remotely.