Gallagher, winner of gold medals at both Beijing 2008 and London 2012, said a mixture of depression, low motivation and expectation of results led him to take the drug.

In a post on Facebook, the Scottish-born 37-year-old confessed to turning to EPO in the build-up to the Australian road national championships and Para-cycling Track World Championships early this year.

Gallagher was provisionally suspended by the Australian Sports Anti-Doping Authority (ASADA) after testing positive for EPO during a training camp in Italy in July.

It means he will not be able to defend the individual pursuit crown he has held since the Paralympics in the Chinese capital. 

"I have done the wrong thing," Gallagher wrote.

"People deserve an explanation and I want to give it.

"Although there are never any excuses for something like this, you do self-analyse and look for reasons.

"No-one in my life saw this coming and if you asked me over a year ago I would never have seen it coming either.

"Worsening mental health issues [depression] and other personal issues in life lead to inability to train and hurt myself like I used to.

"With the expectations of living up to past performances in a Paralympic year and the need to do so to pay my bills the pressure mounted.

"Rather than seek help I self-medicated to motivate, crossed the dark line, took shortcuts and cheated.

"I had many ways of justifying this use for Para-sport which in hindsight were merely just dark, paranoid and selfish justifications to talk myself into it."

Gallagher, a C5 classified athlete, apologised to those affected by his decision and said that nobody knew he was taking EPO.

"I sincerely apologise to my wife, family and close friends," he added.

"People who are standing by me and would have no matter what.

"It seems so stupid now to have not let them in sooner before things got this far."

One of the leading names in his sport, Gallagher also claimed bronze medals in the road race at Beijing 2008 and in the road time trial in London four years later.

His positive test provided a blow to the Australian team here at Rio 2016, which is due to get underway with the Opening Ceremony at the Maracanã Stadium on Wednesday (September 7).

One of Gallagher’s team-mates, seven-time Paralympian Kieran Modra, told The Sydney Morning Herald the news has "really shaken up" the squad. 

"It is hard," Modra, due to compete in the individual pursuit, road race and road time trial at Rio 2016, said. 

"Every time I sit down and relax it always creeps back into your mind again.

"You think of the good times and you miss him, basically.

"You wish he was here."

Australia’s team for Rio 2016 is expected to feature 177 athletes competing in 16 sports.