The US PGA Tour slapped a ban on players who are competing in the Saudi-funded LIV Golf series on Thursday -- just half an hour 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 and with fighter planes circling.

As LIV Golf chief executive Greg Norman looked on, Phil Mickelson, Dustin Johnson and Scott Vincent started their rounds on the first hole.

But the action was quickly overshadowed by a PGA Tour memo to all its members informing them that the players competing at the St Albans tournament without releases 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, two of the biggest names who have signed up to the new series.

Phil Mickelson is the biggest star in the field at the LIV Golf Invitational London
Phil Mickelson is the biggest star in the field at the LIV Golf Invitational London AFP / Adrian DENNIS

LIV Golf immediately responded, calling the PGA's action "vindictive" and saying it "deepens the divide between the Tour and its members".

"This certainly is not the last word on this topic," it said. "The era of free agency is beginning as we are proud to have a full field of players joining us in London, and beyond."

The dispute could be heading for the courtroom if any of the 17 players named by Monahan, 10 of whom had resigned their PGA Tour membership, pursue a legal challenge.

Norman has said previously he is braced for a legal battle, anticipating fierce opposition from the established tours.

As well as Mickelson, LIV Golf players banned by the PGA include Talor Gooch, Matt Jones and Ian Poulter.

Others who had already resigned their PGA Tour membership but were also listed include Johnson, Sergio Garcia, Louis Oosthuizen and Lee Westwood.

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.

Two-time major winner Dustin Johnson is one of the biggest names at the LIV Golf Invitational London
Two-time major winner Dustin Johnson is one of the biggest names at the LIV Golf Invitational London AFP / Adrian DENNIS

Players asked for releases to compete in the LIV event in England but were rejected by the US-based Tour, in part because there was a conflict 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 and two-time major champion Justin Thomas, currently playing in Toronto, supported the action taken by the PGA Tour.

"I have great belief and great confidence in the PGA Tour and where we're going and continuing to grow to, and those guys just aren't going to be a part of it," Thomas said after the first round of the Canadian Open.

McIlroy, runner-up at the Masters in April, said Monahan had simply enforced membership regulations and had "done the right thing".

The DP World Tour, formerly known as the European Tour, has yet to announce what action it will take over its players who are competing in St Albans.

The eight LIV tournaments scheduled for 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, simultaneous "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 an eye-watering fee of $200 million to compete while Johnson is reportedly receiving $150 million in appearance fees.

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 insisted he did not "condone human rights violations", adding 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".