Spieth's withdrawal was confirmed by International Golf Federation (IGF) President Peter Dawson this afternoon.

South Korea's world number 41 K.T. Kim has also pulled-out, along with France's world number 78 Victor Dubuisson.

The latest withdrawals are yet another major blow for a sport which is already facing huge criticism over the absence of many of the world's best players.

Spieth, winner of the Masters and US Open in 2015, had repeatedly claimed to be keen to represent United States in the Brazilian city.

No details have yet been given as to why he changed his mind, but the 22-year-old voiced concerns late last month about the risk of security and Zika virus, the mosquito-borne disease which has been linked with a condition in which babies are born with small heads and underdeveloped brains.

But hardly any athletes in other sports have withdrawn for health reasons, leading many to conclude that golfers are using this an excuse to hide their apathy for competing at the Games.

American cyclist TJ Van Garderen is the only non-golfer to have cited Zika as the reason for his withdrawal, while Lee-Anne Pace is the only female golfer to have pulled-out.

As well as Australia's world number one Day, Ireland's McIlroy, and American duo Spieth and Johnson, other golfers to have opted not to attend are Australia's Adam Scott and Mark Leishmann, South Africa's Brandon Grace, Louis Oosthuizen and Charl Schwartzel, Spain's Miguel Angel Jimenez, Ireland's Graeme McDowell and Fiji's Vijay Singh.

World number five Bubba Watson is now set to lead a US team also consisting of seventh ranked Rickie Fowler, 13th ranked Patrick Reed and world number 15, Matt Kuchar.

"Collectively, the number of [player] withdrawals has not put golf in a good light," said Dawson today.

"It's certainly disappointing.

"I think there's been an over-reaction on the Zika situation."

Defending Open champion Zach Johnson has also questioned the inclusion of golf, football and basketball in the Olympics.

The 40-year-old claims the Olympics are less important to those sports than other events such as majors, in golf, the World Cup, in football, and the National Basketball Association (NBA) league

"I don't know if golf has its place," he said. 

"Basketball and soccer, do they really need to be in there either? 

"My guess is they want a World Cup, an NBA Championship, before a gold medal.

"No offence to the Olympics but I'd rather be on the Ryder Cup team. 

"As an American golfer I have that opportunity and that's what I'd rather do."

Spieth is expected to address this issue in more detail tomorrow.