Golf's ruling bodies, the United States Golf Association and the Royal and Ancient Club, will make a final decision in spring next year on a new Rule 14-1b, which "would prohibit strokes made with the club or a hand gripping the club held directly against the player's body, or with a forearm held against the body".
Harrington, who won the Open in 2007 and 2008 as well as the 2008 US PGA Championship, said: "If belly putters were invented today, they'd definitely be banned, I think everyone agrees on that.
"The question is should they be banned now after 15, 20 years of people using them?
"Twenty years ago you'd hear people like Tom Watson saying 'no, it shouldn't be used', but the biggest issue was whether a guy was going to use it to measure two club lengths out of a water hazard, take a free drop using it. Nowadays, it's become a lot more commonplace and you don't hear that issue whatsoever.
"But the rules of golf have always been that you can't anchor a club to your body, and clearly this is anchoring. There are less variables so it certainly seems to help guys under pressure and it goes against what is seen to be the normal way of playing golf."
"The R&A and USGA have come to the conclusion that if they don't move now, it's becoming so commonplace in the junior game that people are going to think it's normal."
Player, who won all four majors including the Masters and the Open three times apiece, said: "I think the USGA and R&A have been brilliant and I congratulate them.
"I don't think they should be getting any feedback from the players. They are the ruling bodies and they should be able to make the rules as they see fit.
"Obviously a player who is using the long putter is going to want to continue using it."