The PGA Tour has suspended the 17 members who are competing in the inaugural LIV Golf International Series event, it announced Thursday.
Players who resigned their membership before starting the LIV Golf event being held outside London that began Thursday are also no longer eligible to compete in tour events or the Presidents Cup.
Among those now banned from tour events are six-time major champion Phil Mickelson, two-time major champion Dustin Johnson and longtime Ryder Cup participants Ian Poulter, Lee Westwood and Sergio Garcia.
Monahan wrote that any players who take part in future LIV Golf events will face the same punishment.
Mickelson, asked how he felt about Monahan's memo, paused for 10 seconds as he weighed what he was going to say before answering, "Any PGA Tour matters, I'm not going to discuss it publicly at this time."
Poulter, meanwhile, expressed frustration with the PGA Tour's decision and said he would fight the suspension.
The memo said players who compete in LIV events are ineligible to participate on the PGA Tour or any other tours it sanctions, including the Korn Ferry Tour, PGA Tour Champions, PGA Tour Canada and PGA Tour Latinoamerica.
The PGA Tour announced the discipline less than 30 minutes after 17 of its members or former members who resigned from the tour in the past week hit their opening tee shots in the inaugural LIV Golf event at Centurion Club outside London.
Two other past major winners, 2020 U.S. Open champion Bryson DeChambeau and 2018 Masters champion Patrick Reed, have also reached agreements with LIV Golf to compete in future tournaments, sources told ESPN on Wednesday.
Johnson and Garcia are among the players who have resigned from the tour, along with 2010 Open champion Louis Oosthuizen, 2011 Masters champion Charl Schwartzel and Kevin Na.
Graeme McDowell, the 2010 U.S. Open champion, said he resigned as a PGA Tour member about 30 minutes before teeing off Thursday. He said he didn't feel he should have to resign but also expected the punishment that was handed down.
Four-time major winner Rory McIlroy, speaking at a news conference at the RBC Canadian Open in Toronto on Wednesday, said he was concerned about golf's future.