Top investors in Trinity Mirror, which publishes the Sunday Mirror, Daily Mirror and the People, want to know more from its chief executive, Sly Bailey, about phone hacking allegations made last week by a former employee.
Big shareholders, including Standard Life and Aviva, are understood to be making their views known to Trinity's chairman, Sir Ian Gibson, after Bailey launched an inquiry last week into its entire national and regional editorial operations.
The review does not specifically cover hacking, but a company spokesman said: "We can confirm that we're conducting a review of editorial controls and procedures."
On Saturday, the controversy over practices at Trinity intensified after allegations were made on the Guido Fawkes website that money had been paid to a blagger for illegally obtained information while Piers Morgan was editor between 2001 and 2002.
Guido also claims that invoices, totalling £442,000 ($826,000), were sent to the Mirror group over a six-year period for services provided by the blagger. The news will put Bailey under more pressure to explain whether phone-hacking, and other practices, were carried out by her newspaper.