A "killer email" showed that former Daily Mirror editor Piers Morgan knew two of his journalists were engaged in stock market manipulation, the Leveson Inquiry heard.
However lawyers for Trinity Mirror, which publishes the Daily Mirror, described the email as an "urban myth" and said it was never found despite a thorough investigation.
In 2006 financial journalist James Hipwell was jailed for six months and his colleague Anil Bhoyrul was sentenced to 180 hours community service for using the Daily Mirror's City Slickers column to ramp shares in a "tip, buy and sell" scam that netted them thousands of pounds.
Morgan, the then-editor, also made money from shares but was cleared of wrongdoing by a Department of Trade and Industry (DTI) investigation as well as the paper's own internal inquiry.
Seymour said in a written statement to the Leveson Inquiry published today: "There was, in my view, a 'killer' email showing conclusively that the editor knew what was going on.
"This was given to the PCC as part of the evidence handed over yet still it did nothing. A shameful failure of responsibility and duty."
Trinity Mirror's solicitors Herbert Smith responded in a letter to the inquiry: "Trinity Mirror informs us that the existence of such an email was an 'urban myth' during the City Slickers saga.
"No such email was ever found despite a thorough investigation by the DTI - which included, inter alia, the seizure of a number of personal computer hard drives including those of Messrs Morgan, Hipwell and Bhoyrul."
Seymour also said he had no personal knowledge of phone hacking but Morgan had referred to the practice.
He said in his statement: "All I ever heard about it at the Mirror came from the mouth of Piers Morgan, a notoriously unreliable and boastful source.
"In fact, I have reason to believe that one example of phone hacking which he claimed to have been involved in was nothing to do with him or the Daily Mirror."
Herbert Smith said Seymour's witness statement was "not credible" and that "very little weight" should be placed on what he said.