Mickelson made birdie after using his wedge in this Torrey Pines bunker
Lee Westwood has accused Phil Mickelson of "bending the rules" over his use of a controversial 20-year-old club.
Mickelson is competing at the Farmers Insurance Open in San Diego using a Ping Eye 2 wedge which features banned U-shaped grooves to generate more spin.
However, Mickelson's club is within the rules because of a legal loophole which dictates that pre-1990 Ping wedges are permitted in tournaments.
Westwood said: "I don't think it's breaking the rules, it's bending them."
Mickelson's compatriot and fellow professional Scott McCarron had harsher words on the matter.
Speaking during the PGA Tour's Farmers Insurance Open at Torrey Pines, he told Friday's San Francisco Chronicle: "It's cheating, and I'm appalled Phil has put it in play.
"As one of our premier players, he [Mickelson] should be one of the guys who steps up and says this is wrong."
World number two Mickelson initially responded to McCarron's comments by merely stating that he disagreed with the new rules.
However, following his third round at Torrey Pines, he hinted that legal action against McCarron could be a possibility.
"Well, we all have our opinions on the matter, but a line was crossed, and I just was publicly slandered, and because of that I'll have to let other people handle that," Mickelson said.
Westwood, though, speaking at this week's Qatar Masters on the European Tour, stopped short of such a damning indictment.
"It's a very strong word to use, cheating.
"It wouldn't be my choice to use them, but it's obviously not against the rules or else he wouldn't do it.
"I could do it more than anybody else because I've got thousands of Ping wedges. I have the opportunity to do it and I don't."
Mickelson's Ping wedge is allowed by the United States Golf Association and the R&A, and approved by the PGA Tour following a 1993 legal settlement with the club manufacturer.
The ruling takes precedence over the governing bodies' new regulations which favour wedges with V-shaped grooves.
The PGA Tour, meanwhile, has reiterated the legality of the clubs and cautioned against criticism of players who use them.
"Because the use of pre-1990 Ping Eye 2 irons is permitted for play, public comments or criticisms characterising their use as a violation of the Rules of Golf as promulgated by the USGA are inappropriate at best," a statement read.
Mickelson ended round three on nine under, four shots adrift of leader Ryuji Imada of Japan in California.
American Ben Crane and Scottish-born Australian Michael Sim were tied second on 11 under, while England's Justin Rose was four under, Luke Donald two under and Scotland's Martin Laird one under.