In a move that sent shockwaves around the sporting world, FIFA said it was calling Blatter to appear at an ethics hearing on Sunday, three days before the 75-year-old Swiss stands against challenger Bin Hammam in the election for the most powerful job in football.
Bin Hammam will also face the hearing at FIFA house, along with CONCACAF president Jack Warner and two Caribbean officials, and the outcome may determine whether the June 1 election can go ahead as planned.
"I cannot comment on the proceedings that have been opened against me today," Blatter, who has been FIFA president since 1998 and is standing for a fourth term, said in a brief statement.
"The facts will speak for themselves," added the Swiss.
FIFA has been under pressure to clean up its act and become more transparent after two executive committee members were suspended last year after allegedly offering to sell their votes in the 2018 and 2022 World Cup hosting contest to undercover newspaper reporters.
However, few observers could have expected anything as dramatic as this week's events.
"The FIFA presidential election campaign has descended into a farce," Britain's sport minister Hugh Robertson said in a statement. "With both of the candidates having allegations of corruption aimed at them the election should be suspended."
The English FA, angry over its failed bid for the 2018 World Cup, had already announced it would abstain from voting for either candidate.
However, Russian Prime Minister Vladimir Putin, whose country will host the 2018 World Cup, backed Blatter.
"To accuse Blatter of corruption is complete rubbish," Putin told reporters. "If I was voting (in the FIFA election) I would vote for Blatter."
BRIBERY REPORT
The case was opened on Wednesday when CONCACAF secretary general Chuck Blazer, who like Warner and Bin Hammam is a member of FIFA's Executive Committee, reported a possible case of bribery in the election campaign.
Asian Football Confederation president Bin Hammam, 62, has denied any wrongdoing in the matter, which concerns a meeting attended by the Qatari and Warner, who is a government minister in Trinidad & Tobago, on May 10-11.
According to FIFA's statement on Friday, ethics proceedings were opened against Blatter at Bin Hamman's request because the FIFA president may have known about cash payments to delegates at the meeting.
CONCACAF includes 35 of the 208 national associations which make up the FIFA Congress and, alongside Asia, is the only confederation which has not decided who it will back.
Europe, South America, Africa and Oceania have all decided to support Blatter.
Bin Hammam admitted he had paid the expenses of the delegates but said he had done nothing wrong and was victim of a conspiracy.
"It is quite obvious that, following previous failed attempts, this is part of a final effort to prevent Mr. Bin Hammam from running for the FIFA presidency," he said.
"Nobody has ever tried to hide the fact that Mr. Bin Hammam paid for the delegates' travel and accommodation expenses and covered the meeting's administrative costs," said the statement, referring to the CFU meeting.
"At this congress, Mr. Bin Hammam presented his programme, which included proposals to give more say, more pay, more support and more responsibility to the national associations.
The presidential election, in which Blatter and Bin Hammam are the only candidates, is scheduled to take place on June 1 at the annual FIFA Congress in Zurich.
FIFA would not comment on the potential consequences of the ethics committee hearing, to be headed by Namibia's Petrus Damaseb.
