Giving evidence in a room occupied only by lawyers to protect his identity, the former News of the World investigations editor, often referred to as ‘the fake sheikh’ for his use of disguises, told the Inquiry into that his work had led to more than 260 "successful criminal prosecutions", including his most high-profile inquiry into Pakistani cricketers who were subsequently convicted of match-fixing.
Responding to a question over whether he believed that if there was a conflict between what he perceived as a famous person's public persona and the story, then it was worth investigating: "If it's hypocrisy then very much. If they present themselves as wholesome characters and trade on that status then I think it's totally justified."
He said he was unaware of phone-hacking at the paper prior to the arrest of its royal editor Clive Goodman in 2006.
When Mr Mahmood appeared before the Inquiry again on 26 January he admitted to deliberately altering the digital record of a story when he was at The Sunday Times in 1988 to hide a crucial mistake, lied about his cover-up and then resigned before he could be fired.