Police said they're investigating a suspected conspiracy at the company during 2003 and 2004, the Wall Street Journal reported.
Trinity Mirror says the company has been notified of the investigation.
"The group does not accept wrong doing within its business and takes these allegations seriously," Trinity Mirror said in a statement.
Police earlier this year arrested four current and former Trinity Mirror journalists for allegedly intercepting voice mail messages in 2003 and 2004, the Journal said.
The Journal said two of the arrested men were James Scott, the editor of Sunday People, and Nick Buckley, the deputy editor of Sunday People.
The men were not charged with any crimes and released on bail.