Piers Morgan has repeatedly denied any knowledge of phone hacking at the Daily Mirror.
The former editor of the newspaper told the Leveson inquiry into press standards he "had no reason or knowledge to believe it was going on" during his 10 years at the publication, but when asked about one incident he didn't want to answer for fear of "compromising a source".
Piers was asked by the inquiry's lead counsel, Robert Jay, to say who played him a voicemail message left for Sir Paul McCartney's ex-wife Heather Mills.
He explained: "I can't discuss where I was played that tape or who played it, because to do so would be to compromise a source, and I can't do that."
The talk show host was also asked about an appearance on Radio 4 show Desert Island Discs in 2009, on which presenter Kirsty Young questioned him on unethical practices in the industry.
He said: "I didn't hear her say phone-tapping. She rattles off a list of stuff, and if you listen to it in real time I think you would see that."