Once-beaten Anthony Joshua said on Wednesday there is no one to blame but himself for the shock loss of his heavyweight boxing titles to Andy Ruiz Jr last Saturday.

The 29-year-old British pugilist went down to one of the biggest upsets in the history of the heavyweight division at Madison Square Garden with Ruiz sensationally stopping Joshua, the previously unbeaten WBA, IBF, IBO and WBO heavyweight champion.

Joshua, whose camp activated a rematch clause on Tuesday, moved on Wednesday to dispel rumours something had been wrong with him as many pundits had wondered about his listless performance.

Joshua, who first came to public attention in winning Olympic gold in London in 2012, has refused to cut ties with trainer Rob McCracken and accepted the blame lay with him.

But his arch-rival, Dillian Whyte, has once again accused the Olympian of potentially using performance enhancing drugs.

Whyte alleged that Joshua was not as strong in the fight, because of the added anti-doping tests that are conducted in the United States.

He believes Joshua uses PEDs through therapeutic use exemptions.

"Joshua could be heard on camera asking his trainer Rob McCracken, 'Why do I feel like this?' after the sixth round,' Whyte said on Youtube.

"It is because you're in America with the VADA (Voluntary Anti-Doping Agency) testing and you're not on the juice, that's why. It's harder to get therapeutic use exemptions in America than the UK, that's why."

In response to Whyte's allegations, a spokesman for Joshua told The Mirror: "These comments are 100 percent false... Anthony is one of the most tested athletes in sport and has been VADA and UKAD tested in the UK for all of his fights.

"He was tested as much in the States as he has been in the UK for every fight."