Speaking at the Leveson investigation into press ethics and standards, Clifford said that he was aware of several stories which had been held back by tabloid editors in the past few months because they were "frightened" of the inquiry.
He also contradicted claims made by Heather Mills about whether she allowed Piers Morgan to listen to a voicemail left for her by ex-husband Sir Paul McCartney while she was on holiday.
Clifford, who claims to have become aware of mobile phone hacking in early 2000, discussed the details of a £1m settlement he reached with former News International boss Rebekah Brooks after he discovered that the News of the World had hacked his phone.
Clifford said that some of the journalists involved in phone hacking at the now defunct Sunday tabloid were pushed into the criminal behaviour.
"It involved a tiny minority and some of them were forced," he said. "If you don't you're out, you're sacked, you're finished. It was a cancer which is now hopefully being cut out."
Clifford told the inquiry that the public only really became incensed about phone hacking after it was revealed that people like murdered schoolgirl Milly Dowler had been targeted.
"What really got the British public angry was Milly Dowler and the McCanns. They didn't care about the stars, or me, having their phones hacked." he said.
"Most people didn't care. But when they read about Milly Dowler and the McCanns they were shocked and horrified and that had an effect."
Clifford said that the process of journalists listening into phone calls went back years, since the days of Muhammad Ali and Marlon Brando.
However, he said that an effective press watchdog is required to block papers from publishing stories that could be damaging, particularly to "ordinary people".
Clifford was also asked about the infamous "Freddie Starr Ate My Hamster" story, which was published in The Sun in 1986, alleging that the comedian had consumed someone's pet.
He said that he gave the go-ahead for the then-Sun editor Kelvin McKenzie to publish the story as Starr had an upcoming tour at the time and he thought it would be good publicity.
Earlier today, Heather Mills appeared at the inquiry to discuss a dispute over a message left for her in 2001 by then-husband Sir Paul McCartney.
Mills repeated claims she made last August that a journalist had contacted her to say that he knew about the couple's problems, and also mentioned the song sung by Sir Paul in the voicemail.
The controversy came after former Daily Mirror editor Piers Morgan confirmed to the inquiry last year that he had listened to the voicemail message left for Mills by Sir Paul, but refused to say when or where over fears that it would compromise his "source".
Mills has suggested that Morgan could only have heard the voicemail after it had been accessed by the journalist via phone hacking, as she had never given him to access to her voicemails.
But as the statement from Mills was read to Clifford today at the Royal Court of Jusitce, he said that her claims were "totally untrue; 100% untrue; without any truth or foundation at all".
Then he added: "There's an awful lot of things I could say about Heather Mills but I won't."