Two people have died and another has been seriously injured following the collapse of part of the stadium due to host the World Cup opener in Sao Paulo.
A crane fell and destroyed part of the roof covering the north stand fell at the Corinthians Arena, which is due to host the first game of the 2014 tournament on 12 June.
Early reports claimed three people had died but this has since been revised to two, whilst a third person has been rushed to hospital with serious injuries.
A FIFA statement read: "FIFA and the LOC (local organising committee) have learnt of the death of workers at the Corinthians Arena site in Sao Paulo with great sadness.
"We wish to send our heartfelt condolences to the family of the workers who tragically died today.
"The safety of workers is the top priority for FIFA, the LOC, the federal government. We know the safety of all workers has always been paramount for all the construction companies contracted to build the 12 FIFA World Cup stadiums.
"The local authorities will fully investigate the reasons behind such a tragic accident."
Corinthians, the Brazilian football club that will move into the new stadium, confirmed the deaths on their website, in a joint statement with the constructors. "The board of Sport Club Corinthians Paulista hereby deeply regret the accident earlier in Corinthians Arena. No other information at this time.
"Because of the incident that occurred in work at Arena Corinthians on Wednesday afternoon, Sport Club Corinthians Paulista decreed seven days of mourning."
Odebrecht and Corinthians indicated they would prove superficial damage and stressed it was a routine operation that had gone wrong.
"The structure of the grandstand was not compromised," the joint statement said. "It was the 38th time this type of procedure had been performed during the work."
FIFA president Sepp Blatter wrote on Twitter soon after the news broke: "I'm deeply saddened by the tragic death of workers @Corinthians arena today. Our heartfealt condolences are with the families."
The incident is the latest of several at the proposed stadia for the tournament, and once again throws into doubt Brazil's ability to meet FIFA's completion deadline for stadiums to be ready for next summer's tournament.
Work was temporarily halted at the Arena da Baixada stadium in the city of Curitiba last month after a judge ruled there was a serious risk of accidents to workers carrying out building work on the site.
And, a month ago, organisers faced another setback after a fire at the Arena Pantanal further slowed down the development of the 44,000 stadium in Cuiaba.
Sky Sports News reporter Geraint Hughes, who spent time in Brazil for the Confederations Cup last summer, said: "If Brazil wasn't under enough pressure in terms of infrastructure and readiness for the World Cup, it will be now.
"The delivery of the remaining stadiums was due to be December 2013 but FIFA agreed with organisers to push that back to January to give them more time. They were up against it.
"Other social implications have since come to light. We've seen the protests over the cost of living, of bus fares and also the cost of hosting the World Cup. Millions took to the streets of several cities to protest against this."
Brazilian journalist Pedro Redig rejected suggestions the accident posed a threat to the opening game of the finals taking place in the stadium but admitted that the country still had much to do before it was ready to host the tournament.
"It is just one more problem," he said. "Brazil is setting a huge challenge to get this World Cup together.
"There is a lot of infrastructure that still needs to be put in place, regarding airports and transport and facilities."
The Corinthians Arena is scheduled to host four group games at the finals in 2014, along with a round of 16 tie and one of the semi-finals.
The draw for the finals will be made on 6 December.
I'm deeply saddened by the tragic death of workers @Corinthians arena today. Our heartfealt condolences are with the families.- Joseph S Blatter (@SeppBlatter) November 27, 2013