Sebastian Coe has insisted it would be "impossible" for London to step in as 2016 Olympic hosts should the delays affecting the preparations for Rio de Janeiro prove insurmountable.
A senior International Olympic Committee figure has declared that Rio is the worst-prepared of any host city two years out from the Games.
Coe however believes Rio will overcome their difficulties and said that there was no possibility of London stepping into the breach.
He told BBC Radio Five Live: "This is a massive, massive undertaking but Rio will get there.
"Before Sydney (2000) and Athens (2004), they went through exactly the same sort of questioning two years out.
"But it would be impossible for London with just over two years' notice to say let's bring it all back. That's not going to happen.
"The reality is a lot of our venues don't exist any more.
"The Olympic Stadium is now being converted into a football and track and field venue.
"We used a lot of temporary venues and they of course disappear and not to mention the organising committee, they have all gone off to do other things."
IOC vice-president John Coates said last month the situation in Rio "was critical on the ground".
Coates, who has been involved in the Olympics for 40 years, has made six trips to Rio as part of the co-ordination commission responsible for overseeing the preparations.
He said: "It's the worst that I've experienced. 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.
"It is unprecedented for the IOC but there is no Plan B. We are going to Rio."