If there is enough evidence of wrongdoing, FIFA has the power to strip the World Cup from both Russia and Qatar.
Soccer's governing body, currently engulfed in the worst corruption scandal in its 111-year history, has a provision which allows it to revoke hosting rights for ''unforeseen contingencies and force majeure.''
''If there is still evidence that a substantial amount (of votes) have been bought by illegal means, then of course it could be changed,'' Swiss law professor and anti-corruption expert Mark Pieth told The Associated Press. ''I would not totally rule it out, and of course there are many other questions linked to it.''
So far, the Russians and Qataris don't seem to be worried.
FIFA has been plunged into crisis since seven officials were arrested in dawn raids last week at a luxury Zurich hotel ahead of the FIFA congress. They were among 14 indicted by U.S. authorities on corruption charges.
In a separate probe, Swiss authorities are investigating the 2018 and 2022 World Cup bidding contests, which went to Russia and Qatar.
In the wake of the two investigations, FIFA President Sepp Blatter announced his decision to resign. That came only four days after he was re-elected for a fifth, four-term.
U.S. authorities are also looking specifically at the 79-year-old Blatter, but he has not been formally charged.
Russia was awarded the 2018 World Cup and Qatar was given the 2022 tournament in an executive committee vote in Zurich in December 2010. Accusations of vote-buying followed quickly, and FIFA appointed American lawyer Michael Garcia as the lead investigator into possible wrongdoing. Garcia submitted his report late last year, but soon resigned after a disagreement over what was in the file.
Garcia's full report has never been released, but FIFA turned over the document to the Swiss authorities who are still investigating possible corruption.
Several members on that 24-man executive committee have been largely discredited since the vote, including a life ban for Qatari official Mohammed Bin Hammam and some resignations and shorter bans for others.
Besides the Swiss investigation, Australia has added to the pressure with its own police investigation. Australia was one of the losing bidders in the 2022 contest.
FIFA has made late changes to big tournaments before, though not on the same scale. Colombia withdrew as the host of the 1986 World Cup four years before kickoff because of economic problems and Mexico stepped in. The 2003 Women's World Cup was switched from China to the United States with only six months' notice because of the outbreak of the SARS virus.
England, Germany and the United States could be potential replacement hosts for the next two World Cups.
The FIFA executive committee - the powerful panel of officials that awarded the World Cup to Russia and Qatar in the first place - would likely have the final say on where the next two World Cups will be played.
And the position of some of the top executives and possible contenders to succeed Blatter as president are complicated, especially when it comes to Qatar's bid, the most contentious.
Michel Platini, the president of European body UEFA and a stern critic of Blatter, voted for Qatar. Africa soccer confederation president Issa Hayatou, the most senior executive after Blatter, has close links with Qatar. And Prince Ali bin Hussein of Jordan, who lost to Blatter in last week's election, has consistently said Qatar deserves to host the first World Cup in the Middle East.
The Qataris have been saying little since the FIFA crisis erupted, but claim they were cleared by FIFA after Garcia's report was submitted.
''The results of his investigation showed there was no suspicion,'' Qatari foreign minister Khalid bin Mohammad al-Attiyah said.
FIFA ethics judge Joachim Eckert, who assessed Garcia's report, cited ''problematic facts and circumstances'' about the Qatar bid and other candidates but said the evidence was not enough to compromise the bidding process as a whole.
The Russians have criticized the United States for its part in the investigations, with President Vladimir Putin saying the American authorities were meddling in soccer affairs.
''There is a persecution going on with a maniacal desire to accuse Russia of some improbable crimes which then force others to decide to take the World Cup away from us,'' Russian deputy sports minister Natalya Parshikova said.
The United States was another losing bidder in the vote for the 2022 tournament.
Russia has spent much of its $12.4 billion budget, with three stadiums completed and nine more in the works. Qatar has five stadiums in progress and reportedly a $200 billion budget.
The deciding factor on a revote, however, may still be in the hands of the investigators.
''What would be awkward and could happen is that you have evidence two or three votes have been bought, and is that sufficient?'' Pieth said.
Imray reported from Cape Town, South Africa. AP Sports Writers Graham Dunbar in Geneva, James Ellingworth in Moscow and Rob Harris in Berlin contributed to this report.