Phil Mickelson has raised eyebrows at Augusta on the eve of the Masters by accusing some of his rivals of cheating.

The three-time Masters champion said a number of players on the PGA Tour are "intentionally loose" in marking their ball.

Mickelson made the claim during his pre-tournament press conference at Augusta National after being asked his opinion on the four-shot penalty incurred by Lexi Thompson in last week's ANA Inspiration.

Thompson was leading by three shots during the final round when she was informed by tournament officials she would be penalised for an incident which took place the previous day.

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The 22-year-old was penalised two strokes for incorrectly replacing a marked ball on the 17th green and another two for signing an incorrect scorecard, after a television viewer had alerted LPGA officials to the incident via email.

"Rather than address that specific instance, what I would say is this - I know a number of guys on tour that are loose with how they mark the ball and have not been called on it," Mickelson said.

"I mean, they will move the ball two, three inches in front of their mark and this is an intentional way to get it out of any type of impression and so forth - and I think that kind of stuff needs to stop.

"But I think it should be handled within the tour. I think that the tour should go to those players and say, 'look, we've noticed you've been a little lax in how precise you've been in marking the ball. We'd like you to be a little bit better at it' and see if that doesn't just kind of fix the thing.

"Because we've all marked the ball imprecisely, especially when you're standing on the side of the ball like she (Thompson) was and not directly behind the ball, in line with the hole, where it's easy to draw a line."

espite such comments, Mickelson also said Thompson, who went on to lose a playoff to Korea's So Yeon Ryu, should be given the trophy.

"To have a tournament be decided like that, with all the scenarios going around, as far as viewers calling in, as far as it being a one-foot putt with really no advantage, just a little bit of loose marking, if you will, something that happens all the time, intentionally and unintentionally ... I think it should be reversed," Mickelson said.

"I think that she should be given the trophy. I feel like we've all kind of been a little lax at times in the markings of our golf ball and I hate to see it cost somebody a major championship because of that."

Jack Nicklaus offered a suggestion that penalties shouldn't be assessed after a round is completed. "Once the round is over, and the scorecard is signed, the day is over," Nicklaus said Tuesday. "That's my opinion."