Trinity Mirror sought the written confirmations from its current editorial executives, but did not ask for similar guarantees from its former employees, such as ex-Daily Mirror editor Piers Morgan, who has faced allegations in recent weeks over his knowledge of phone hacking practices.
Sly Bailey, the chief executive of Trinity Mirror, said that the publisher had made the move to put to bed "completely unsubstantiated" allegations of malpractice by journalists working for the company.
"The company has sought and received formal written confirmation from senior editorial executives across both the nationals and regional [newspapers], that since the commencement of the Regulation of Investigatory Powers Act in October 2000 and whilst an employee of the group they have not, nor to their knowledge have any of their staff or anyone on their behalf, intercepted any telephone messages, made payments to serving police officers or accessed the police national computer," Bailey told The Guardian.
She added: "It is just good corporate governance. Following the disclosure of the activities of certain journalists at the News of the World, the government has asked Lord Justice Leveson to hold an inquiry into various matters including the regulation of the press."
National newspapers have been under scrutiny ever since a series of shocking allegations of phone hacking at the News of the World, which prompted Rupert Murdoch to shut down the 168-year-old Sunday tabloid.
Last month, the BBC's Newsnight programme revealed evidence apparently linking the Sunday Mirror to the hacking controversy, with a former journalist at the paper claiming to have witnessed routine voicemail interception on a daily basis.
However, much of the focus on Trinity Mirror's involvement in the scandal has been on Morgan, who edited the Daily Mirror for a decade until 2004.
Heather Mills said last week that she was told by a journalist working for Mirror Group Newspapers that a voicemail left for her by ex-husband Sir Paul McCartney had been hacked.
Morgan, who was editor of the Daily Mirror at the time of the alleged incident, has consistently denied ever having hacked a phone, ordered a journalist to do so, or published a story obtained using the practice.
Bailey said that Trinity Mirror was not able to seek assurances from people who were no longer employed by the company, such as Morgan.
She said: "We are not in a position to compel people who no longer work for us. There have been a number of completely unsubstantiated allegations made against our titles with no evidence to support them."
According to the latest circulation figures, the Sunday Mirror has benefitted from the News of the World closure, increasing its circulation in July by 55% year-on-year to 1.78 million.