International Olympic Committee (IOC) vice president John Coates has called Brazil's preparations for the 2016 Rio de Janeiro Games "the worst" he has ever seen.
He says the preparations are critically behind schedule, but warned there was no "plan B" to find another host.
Speaking at an Olympic forum in Sydney, Mr Coates told delegates that construction had not commenced on some venues, infrastructure was significantly delayed and water quality was also a major concern two years out from the Games.
He described the situation as worse than Athens 2004, referring to preparations for the 2004 Games, which were plagued by construction delays.
"It's the worst that I've experienced," he said. "We have become very concerned. They are not ready in many, many ways. We have to make it happen and that is the IOC approach. You can't walk away from this."
The first Games on the South American continent have been hampered by delays, rising costs and poor communication between different levels of the Brazilian government and organisers, prompting criticism from international sports federations.
Organisers of the event did not respond immediately but Rio's mayor Eduardo Paes says the city "has to accept the criticism and work hard.
Mr Coates, who was involved in the organisation of the Sydney 2000 Games as head of the Australian Olympic Committee, said it was proving difficult for the IOC to get the answers they needed.
"No-one is able to give answers at the moment," he said.
"Can they use the car parks in the village for recovery centres? What will be the time to take from this venue to this venue?
"All of those things, they're being fobbed off."