Blatter, who has led football's world governing body since 1998, is being investigated by US prosecutors and the Federal Bureau of Investigation, a person who spoke on condition of anonymity told Reuters late on Tuesday. An FBI spokesman declined comment.

News of Blatter's investigation was earlier reported by The New York Times and ABC News. Blatter has not been charged with any wrongdoing. FIFA did not respond to a request for comment on Blatter being under investigation.

Blatter, a Swiss national who has been a dominant presence at FIFA for decades, announced his decision to step down at a hastily arranged news conference in Zurich, six days after police raided a hotel in the city and arrested several FIFA officials — and just four days after he was re-elected to a fifth term as FIFA president.

He said an election to choose a new president would be held as soon as possible, though a FIFA official said it would probably not take place until at least December.

"FIFA needs profound restructuring," Blatter said. "I decided to stand again to be elected because I was convinced it was the best option for football. Although the members of FIFA gave me a new mandate, this mandate does not seem to be supported by everyone in the world."

Critics, sponsors welcome move

Blatter's decision to step down as FIFA is mired in the worst crisis in its history was welcomed by his critics.

European football federation chief Michel Platini, a French former international player and favorite to succeed Blatter as FIFA president, said: "It was a difficult decision, a brave decision, and the right decision."

The second favorite on the list, Jordan's Prince Ali bin Al Hussein, who withdrew from last week's election after winning 73 votes to Blatter's 133 in the first round, stopped short of confirming he would run again. Asked if there should be a fresh start at FIFA, he told Britain's Channel 4 News: "I'm willing to help."

Greg Dyke, chairman of the English Football Association and one of Blatter's most outspoken critics, said it was "good news for world football," but then questioned Blatter's motive.

"Who got him? Who shot him?" he asked. "I don't believe he went for any sort of moral basis so something has happened between then and now which means he has to resign."

New Zealand Football chief executive Andy Martin told Reuters that Blatter's resignation would help football rebuild its tattered reputation.

"This has lifted a cloud and taken away a lot of the concerns of stakeholders and their association with the sport," he said. "We now want a strong collaborative leader who can bring the football world together and can bring out the change that the game has been crying out for."

The Asian Football Confederation (AFC), which has been a staunch ally of Blatter, said on Wednesday it was monitoring the situation and would discuss internally the "best way forward for both FIFA and world football."

FIFA was stunned last week by the announcement of a US investigation into alleged widespread financial wrongdoing stretching back more than two decades. Swiss authorities also launched their own criminal probe into the awarding of the 2018 and 2022 World Cups to Russia and Qatar, respectively.

Russian Sports Minister Vitaly Mutko said Blatter's decision to step down was "courageous" and would help prevent a split in FIFA.

While Blatter was not mentioned in the US or Swiss investigations, there had been widespread calls for him to quit, mostly from Western nations. Some major sponsors also expressed misgivings about the impact of the scandal.

Coca-Cola and Adidas welcomed Blatter's resignation. Another World Cup sponsor, South Korean carmaker Hyundai Motor, said the move was "a positive first step."

The US Justice Department and the US Attorney's office had no immediate comment. The office of the Swiss Attorney General, which is investigating alleged criminal mismanagement and money laundering at FIFA, said Blatter's resignation would have no effect on its proceedings. It said Blatter himself was not subject to its investigation.

New faces, fresh start

Besides Platini and Prince Ali, several other candidates may emerge in the election for a new president, including Domenico Scala, independent chairman of the audit and compliance committee of FIFA.

Former Brazil international Zico, 62, did not rule out a bid for the presidency, and Venezuelan President Nicolas Maduro suggested Argentine soccer legend Diego Maradona could be the next FIFA chief.

European sports officials said Blatter's resignation was an important move, but that FIFA needed deeper changes.

"Beyond the people, structural reforms must be undertaken," said French Sports State Secretary Thierry Braillard.

Kalusha Bwalya, Football Association of Zambia president and former African Footballer of the Year, said he was shocked.

"The man has done a lot for FIFA," he said. "For Africa he was always there, he was always caring."

The investigation closed in on Blatter on Tuesday, when FIFA denied that his right-hand man, Secretary-General Jerome Valcke, was implicated in a $10 million payment relating to the 2010 World Cup in South Africa that is at the heart of the US case.

At the same time, a letter addressed to Valcke from the South African Football Association was published outlining the transaction.

Hours later FIFA called the emergency news conference and Blatter, who became FIFA secretary general in 1981 and president 17 years later, announced his decision.

Reuters