The chief of the joint Spanish-Portugal bid to stage the 2018 World Cup on Thursday denied allegations of collusion with 2022 candidate Qatar in the bidding process.
British newspaper The Sunday Times ran a report and video on Sunday by undercover journalists posing as lobbyists, which included reported remarks by ex-FIFA general secretary Michel Zen-Ruffinen about Spain-Portugal and Qatar's bids to host the World Cup in 2018 and 2022.
FIFA on Monday said it had asked the newspaper to provide more evidence for a probe into the scandal.
"Last week, the FIFA's ethics committee sent us a letter to inform us that it had decided to open an investigation as the Iberian candidature was suspected of having reached a deal with Qatar," the head of the Spain-Portugal bid, Miguel Angel Lopez, told AFP.
"We are clean," he said. "There is no deal with any candidate at all."
The affair is part of a wider scandal involving alleged corruption in the World Cup bidding process.
FIFA has already provisionally suspended two senior officials and launched its own investigation over alleged World Cup vote-selling.
FIFA's executive committee is to choose the winning host countries for 2018 and 2022, with an announcement due on December 2.
England, Russia and joint bids by Spain-Portugal and Netherlands-Belgium are in the running to host the 2018 World Cup.
For 2022, the contenders are Australia, Japan, Qatar, South Korea and the United States.