Phone hacking appeared to be a "bog-standard journalistic tool" for gathering information, a former Daily Mirror financial reporter has said.
James Hipwell, who was jailed in 2006 for writing about firms whose shares he owned, said he witnessed repeated privacy infringements at the paper.
He told the Leveson Inquiry into media ethics that he overheard showbiz journalists openly talking about it.
Publisher Trinity Mirror has insisted its journalists work within the law.
It has also said they work within the Press Complaints Commission's (PCC) code of conduct.
But Mr Hipwell told the inquiry he had never been given a copy of the code during his time at the paper, under the editorship of Piers Morgan.
He said he never heard reference to the code, and said there were no visible signs of ethical leadership.
In a statement read to the inquiry, he stated: "I witnessed journalists carrying out repeated privacy infringements using what has now become a well-known technique - to hack into the voicemail systems of celebrities, their friends, publicists and public relations executives.
"The openness and frequency of their hacking activities gave me the impression that hacking was considered a bog-standard journalistic tool for gathering information."
Mr Hipwell said he sat next to the showbiz team, where hacking took place daily.
"Showbiz hacks discussed techniques and products of hacking openly," his statement said.
James Hipwell: "Very unlikely Piers Morgan did not know hacking was going on".
"I would go as far as to say it happened every day. It became apparent that a great number of stories... would come from that source."
He described Mr Morgan as "very hands-on" and the "beating heart" of the newspaper, and said the editor spent half an hour every day discussing the showbiz reporters' stories.
"Showbusiness is very close to his heart... and a lot of people who had worked on the showbusiness desk had come from the Sun and they were old friends," he said.
"Nothing really happened on that desk without Piers knowing about it."
Mr Hipwell said he did not report that he had witnessed hacking to Mr Morgan because senior editors accepted it.
He added that it was "very unlikely" Mr Morgan did not know that Mirror journalists were hacking.
But in evidence to the inquiry on Tuesday, Mr Morgan firmly denied any knowledge of hacking under his editorship.
He said he had no reason to believe it was going on.
Mr Hipwell was jailed for purchasing low-priced stocks and then recommending them to readers in the paper's City Slickers column, selling them as their values soared.
He received a six-month prison sentence in February 2006 for pocketing nearly £41,000.