It was "very unlikely" that former editor Piers Morgan did not know about alleged phone hacking at the Daily Mirror, the Leveson Inquiry heard today.
James Hipwell, a former financial reporter for the newspaper, told the investigation into press standards that "there wasn't very much he didn't know about".
He also said that Mr Morgan's phone had been hacked by a colleague.
In his evidence yesterday, Mr Morgan said he was unaware of any phone hacking at the Daily Mirror under his leadership and had "no reason" to believe that hacking was going on.
However Mr Hipwell, who was jailed for purchasing low-priced stocks and then recommending them to readers, told the inquiry today: "Looking at his style of editorship, I would say it was very unlikely that he didn't know what was going on because, as I have said, there wasn't very much he didn't know about.
Mr Hipwell was given a six-month prison sentence in February 2006 for pocketing nearly £41,000. He mentioned the stocks in the Daily Mirror's City Slickers column and then quickly sold them as values soared.
On the subject of phone hacking, Mr Hipwell said: "I would go as far as to say that it happened every day and that it became apparent that a great number of the Mirror's showbusiness stories would come from that source. That is my clear memory."
The witness said he heard one reporter claim they had deleted someone else's voicemail message so that a Sun journalist could not listen to it as well.
"One of the reporters showed me the technique, giving me a demonstration of how to hack into voicemails," he told the inquiry.
Mr Hipwell, who worked at the newspaper between 1998 and 2000, said: "The openness and frequency of their hacking activities gave me the impression that hacking was considered a bog-standard journalistic tool for getting information."
He said hacking seemed to be "perfectly acceptable" to some of the Mirror's senior editors,
Mr Hipwell told the inquiry the reporters believed hacking was acceptable as celebrities were "fair game".
"I think it was seen as a slightly underhand thing to do but not illegal," he said.
"I don't think the illegality of it was ever even considered.
"It just seemed to be fair game, fair play, any means to get a story."
He added: "It became a daily part of their news-gathering operation."
Mr Morgan was an "extremely hands-on" editor who demanded to know where every story had come from, Mr Hipwell went on.
"The newspaper was built around the cult of Piers," he said.
Nothing happened there without Mr Morgan knowing about it, he told the hearing.
"He wanted to know about the details of each story, especially if they were celebrity stories, and he wanted to know where they came from, how do we know, what's the evidence?" he said.
Reporters kept few secrets from their editor and there was no pulling the wool over anyone's eyes, he added.
Mr Hipwell said he had even seen another reporter - a "sympathetic colleague" - hack into Mr Morgan's own phone during the time he himself was under fire for the City Slickers affair.
"I don't think he clinched a great deal of information but he certainly tried," he said.
"Perhaps there wasn't a message there but he did use the technique to hack into Mr Morgan's phone at the beginning of 2000."
Mr Hipwell also suggested it was inconceivable that hacking only took place at the News of the World as there was so much movement of showbusiness journalists between tabloids.
"I've always thought it was nonsense to suppose that phone hacking at the News of the World was an isolated incident at that newspaper, given that some journalists at the News of the World ended up on other newspapers - newspapers that are part of Trinity Mirror," he said.
Discussing the City Slickers incident for which he was jailed, he alleged that his editor had been as guilty as he was but had got away with it.
"I understand why people think I have an axe to grind against him but it's my contention that neither Trinity Mirror nor Mr Morgan took their responsibility for what happened.
"So, yes, I did trade on the same information he did as far as some of the companies went. His investments were much larger than anyone else's on the newspaper."
He admitted he had got carried away and said he was "extremely sorry and regretful that that happened".
Mr Hipwell said he had not really thought about "the ethics of what we were doing".
He said: "Certainly there was no ethical management from our superiors on the newspaper."
Desmond Browne QC, counsel for Trinity Mirror, rejected Mr Hipwell's evidence but said he did not want to hold up the progress of the inquiry by entering into an extensive cross-examination of him.
He added: "He is, on his own account, an acknowledged liar."
The inquiry also heard today from journalists covering Madeleine McCann's disappearance in Portugal.
Daily Express reporter David Pilditch said journalists covering the investigation were under great pressure to bring in stories although it was "near impossible".
The inquiry was adjourned until January 9.