The US PGA Tour slapped a ban on players who are competing in the rebel Saudi-funded LIV Golf series on Thursday -- just 30 minutes after the inaugural event teed off outside London.
The richest tournament in the sport's history -- with a purse of $25 million -- got under way at the Centurion Club in St Albans after a trumpet fanfare.
As LIV Golf chief executive Greg Norman looked on, Phil Mickelson and Dustin Johnson began their rounds on the first hole during a "shotgun start", in which all players start at the same time on different holes.
But the action was quickly overshadowed by a PGA Tour memo to its members informing them that those competing without releases at the event had been suspended.
"These players have made their choice for their own financial-based reasons," wrote Tour commissioner Jay Monahan.
"But they can't demand the same PGA Tour membership benefits, considerations, opportunities and platform as you. That expectation disrespects you, our fans and our partners."
The banned players include six-time major champion Mickelson and former world number one Johnson.
LIV Golf called the PGA's action "vindictive", saying it "deepens the divide between the Tour and its members".
The dispute could be heading for the courtroom if any of the 17 players named by the PGA, 10 of whom had already resigned their Tour membership, pursue a legal challenge.
The golfers who had already resigned included Johnson, Sergio Garcia, Louis Oosthuizen and Graeme McDowell.
That group of players will be removed from the FedEx Cup points list following the end of this week's Canadian Open and will not be able to compete on the Tour as non-members via a sponsor exemption or any other eligibility category.
On the course, Mickelson and Johnson both carded one-under-par 69s to trail leader Charl Schwartzel, the 2011 Masters champion, by four shots.
Mickelson said he was "not going to discuss publicly" PGA Tour matters but McDowell said the situation was not healthy for the sport.
"We're here because we believe we're independent contractors... we should be allowed to compete and play where we want to all over the world," he said.
Ryder Cup star Ian Poulter plans to appeal against the ban after carding a 75 at the 54-hole event -- LIV is 54 in Roman numerals.
"It makes no sense," he said. "Having two tour cards and the ability to play golf all over the world, what's wrong with that?"
- Tournament clash -
Players asked for releases to compete in the LIV event in England -- one of eight scheduled for this year -- but their requests were rejected by the US-based PGA Tour, partly because there was a clash with the Canadian Open.
Monahan warned any players who took part in future LIV Golf events would face the same fate, with multiple reports that US stars Bryson DeChambeau and Patrick Reed plan to sign up.
Four-time major winner Rory McIlroy, currently playing in Toronto, said Monahan had simply enforced membership regulations and had "done the right thing".
The DP World Tour, formerly the European Tour, has yet to react over its players.
The United States Golf Association said earlier this week that LIV players would be able to play in next week's US Open.
The eight LIV tournaments this year, bankrolled by Saudi Arabia's sovereign wealth fund, are worth an eye-watering combined $255 million.
Organisers have pledged to "supercharge" golf, offering 54-hole tournaments with no cuts, "shotgun starts" and a team element.
Players have faced tough questions over Saudi Arabia's human rights record and have been accused of greed in chasing the huge rewards on offer.
Mickelson did not dispel rumours he was receiving a fee of $200 million to compete, while Johnson is reportedly getting $150 million.
Mickelson, who had not played since the publication of comments in February in which he criticised the PGA Tour and LIV Golf's Saudi backers, was grilled by reporters on the eve of the tournament.
The 51-year-old American insisted he did not "condone human rights violations", adding that golf could be a force for good.
But Amnesty International renewed its call for players to speak out about "human rights abuses" in Saudi Arabia, rather than being "willing stooges of Saudi sportswashing".