Their decision to back the established US circuit is a major blow to the upstart project fronted by former world number one Greg Norman.

Four-time Major winner Rory McIlroy said the idea was "dead in the water".

"Who's left? Who's left to go? I mean, there's no one. It's dead in the water in my opinion. I just can't see any reason why anyone would go," McIlroy, 32, said.

"No one really knew where Bryson stood. I was really glad to see DJ and Bryson put out those statements this week. We all want to play against the best players in the world and they're certainly two of the best players in the world."

Those who join the new league could be banned from the PGA Tour, DP World Tour and Ryder Cup.

Former world number one Johnson, 37, is a two-time winner of the Saudi International tournament and had been closely linked with the proposed league.

But the 2020 Masters champion has quashed rumours he was about to defect, saying he was "fully committed to the PGA Tour."

"Over the past several months, there has been a great deal of speculation about an alternative tour; much of which seems to have included me and my future in professional golf," Johnson said in a statement issued through the PGA Tour.

"I feel it is now time to put such speculation to rest. I am grateful for the opportunity to play on the best tour in the world and for all it has provided me and my family.

"While there will always be areas where our tour can improve and evolve, I am thankful for our leadership and the many sponsors who make the PGA Tour golf's premier tour."

DeChambeau, 28, had been rumoured to have been offered more than $100m (£73.6m) to be the figurehead for the Saudi-backed league.

But soon after Johnson's statement the former US Open winner also distanced himself from the project on social media.

"While there has been a lot of speculation surrounding my support for another tour, I want to make it very clear that as long as the best players in the world are playing the PGA Tour, so will I," DeChambeau tweeted.

"As of now, I am focused on getting myself healthy and competing again soon. I appreciate all the support."

DeChambeau, Johnson and Phil Mickelson were expected to be the top names who would lead the breakaway.

European Ryder Cup stars Lee Westwood, Ian Poulter and Graeme McDowell are also thought to be considering offers to join.

However, Johnson and DeChambeau's rejection of the project is a significant setback.

Major-winning stars including McIlroy, world number one Jon Rahm, Open champion Collin Morikawa and Justin Thomas have already said they are not interested.

Last week it was claimed 20 leading players had already signed up, but now a breakaway league seems a much more distant prospect.