Phil Mickelson watches his shot from the 1st tee on the first day of the LIV Golf Invitational Series event at The Centurion Club in St Albans on Thursday. AFP

PGA Tour Commissioner Jay Monahan sent a letter to members not long after play in the lucrative breakaway series began outside London to address the 17 golfers who he said "decided to turn their backs" on the US-based circuit.

"These players have made their choice for their own financial-based reasons. But they can't demand the same PGA Tour membership benefits, considerations, opportunities and platform as you," Monahan wrote. "That expectation disrespects you, our fans and our partners.

"You have made a different choice, which is to abide by the Tournament Regulations you agreed to when you accomplished the dream of earning a PGA Tour card and – more importantly – to compete as part of the preeminent organization in the world of professional golf."

Mickelson, who counts six major championships among his 45 PGA Tour wins, former world number one Dustin Johnson and 2017 Masters winner Sergio Garcia are among the higher-profile golfers in the 48-player field at Centurion Club this week.

According to the memo, players who join the LIV series that threatens to blow apart men's professional golf, are no longer eligible to participate in PGA Tour events, including the Presidents Cup and all other tours sanctioned by the circuit.

Even if players resigned from the tour ahead of the first LIV Golf event, as Johnson announced this week, they will not be permitted to play as a non-member via a sponsor exemption or any other eligibility category.