Spain, Portugal and Qatar are reportedly being investigated in connection with alleged collusion over voting for the 2018 and 2022 World Cups.
The BBC reports the three countries are being investigated as part of a wider inquiry by football's world governing body into alleged breaches of the bidding rules.
Spain and Portugal are making a joint 2018 bid, while Qatar is eyeing 2022.
On Thursday the chief of the Portuguese Football Federation Gilberto Madail rejected allegations of any collusion with other candidates.
Collusion between bidding countries is explicitly forbidden by the regulations, but FIFA chiefs have ruled out postponing the 2 December vote for the 2018 and 2022 World Cup hosts.
Russia and England are facing joint bids from Spain/Portugal and Netherlands/Belgium in the race for the 2018 tournament.
The United States, Japan, South Korea, Qatar and Australia are competing to hold the event in 2022.
Fifa's investigation will run alongside the ethics committee's probe into two senior Fifa officials who have been provisionally suspended after being caught up in a World Cup cash-for-votes scandal.
Reynald Temarii and Amos Adamu were secretly filmed by a British newspaper allegedly offering to sell their votes on the bidding to host the 2018 and 2022 World Cup tournaments.
Mr Temarii, the president of the Oceania Football Confederation, and Amos Adamu from Nigeria both deny wrongdoing.
FIFA's ethics committee is to report in November.