Former world number one Dustin Johnson said Tuesday he has resigned from the US PGA Tour to play in the Saudi-funded LIV Golf Invitational Series, which tees off outside London this week.

The decision effectively rules the American two-time major winner out of participating in the Ryder Cup, which pits the United States against Europe every two years.

Johnson was speaking at a press conference at Centurion Club ahead of the inaugural event in the series, which starts on Thursday.

The world number 15 had said in February he was committed to playing on the PGA Tour, which has refused releases for members to play in the LIV Golf opener -- which will clash with the Canadian Open.

But Johnson has now quit, following a similar decision by veteran US golfer Kevin Na, who is also in the field for the event in St Albans.

The 37-year-old, who has won more than $74 million on the PGA Tour, said: "Right now, I've resigned my membership from the Tour."

Johnson, who is reportedly receiving $150 million in appearance fees to play in the new series, said it was difficult to predict the consequences of his decision.

Dustin Johnson speaks to the press at the Centurion Club, near London
Dustin Johnson speaks to the press at the Centurion Club, near London AFP / ADRIAN DENNIS

When asked whether he had opted for money over his country, he said he had chosen "what's best for me and my family".

"The Ryder Cup is unbelievable and it's something that has definitely meant a lot to me," added Johnson, who won all five of his matches in the United States' record 19-9 win over Europe at Whistling Straits last year.

"I'm proud to say that I've represented my country and hopefully I will get a chance to do that again, but I don't make the rules."

One of the uncertainties has been whether players who split with established tours to join LIV Golf -- which does not currently offer ranking points -- would still be allowed to play in golf's four majors.

Former Masters and US Open champion Johnson can still play in the tournaments, which are separately governed, due to his stellar record.

The United States Golf Association confirmed on Tuesday that players taking part in the LIV opener would be free to play in next week's US Open in Brookline, Massachusetts.

Dustin Johnson practises on the driving range at the Centurion Club in St Albans, north of London
Dustin Johnson practises on the driving range at the Centurion Club in St Albans, north of London AFP / ADRIAN DENNIS

It has been reported that Spain's Sergio Garcia and South Africans Louis Oosthuizen, Charl Schwartzel and Branden Grace, who are in the 48-man field for the opening LIV event, have also resigned from the PGA Tour.

Six-time major winner Phil Mickelson confirmed on Monday he would play in St Albans in a major coup for the organisers.

Mickelson has not played since the publication of comments in February, in which he criticised the PGA Tour and LIV Golf's Saudi backers.

The PGA Tour has warned of disciplinary action for players who play in the LIV series but the DP World Tour, formerly the European Tour, has been more opaque, saying it was "evaluating each request on a case-by-case basis".

The new series, which comprises eight tournaments this year, is bankrolled by Saudi Arabia's sovereign wealth fund.

Players will compete as individuals and teams for eye-watering purses of $25 million in all seven regular-season events, played over 54 holes with no cut.

The eighth and final event will be a team championship, with a total prize fund of $50 million.

Amnesty International has said the series is an example of Saudi Arabia attempting to "sportswash" its human rights record.

The golfers were questioned on the human rights issue on Tuesday, including the 2018 murder of journalist Jamal Khashoggi.

A US intelligence assessment found that the Gulf kingdom's de facto ruler, Crown Prince Mohammed bin Salman, "approved" an operation to capture or kill Khashoggi, a columnist who was critical of Saudi's rulers.

Saudi officials deny this and say that his killing in the kingdom's Istanbul consulate -- which sparked worldwide outrage -- was a "rogue" operation.

"The Khashoggi situation, we all agree that was reprehensible," 2010 US Open champion Graeme McDowell told reporters on Tuesday.

"No-one's going to argue that fact but we're golfers. We are not politicians. I know you guys hate that expression, but we are really not, unfortunately. We are professional golfers.

"If Saudi Arabia wanted to use the game of golf as a way for them to get to where they want to be and they have the resources to accelerate that experience, I think we are proud to help them on that journey using the game of golf."