We'll never be able to stop the cheats, says FIFA boss Sepp Blatter
He also said that it would be impossible to stop them after revelations of a global match-fixing scandal this week.
European police revealed on Monday that about 680 suspicious matches, including qualifiers for the World Cup and European championship and the Champions League, had been identified in a match-fixing inquiry.
He said the allegation of match-fixing and manipulation of soccer results were not new.
"We know that there are games that are manipulated and we also know that it is very, very difficult to get the organisations (behind them) and especially the cheats," Blatter said in French.
"This is a serious danger. If games are rigged, there is no more interest in watching football," he said.
However, Blatter added that most of the matches cited in the European investigation had already been uncovered and some of the cases were in court.
The allegation is the latest in a series of corruption scandals that have wracked the world soccer body.
FIFA said on Wednesday it had launched a web page to enable individuals to make anonymous reports of corruption by officials and attempts at match-fixing.
But Blatter said even FIFA's "early warning system" was far from effective in curbing the problem and there was no legislation to punish the people behind the scams that are threatening the global game.
"There is virtually no legislation that allows countries to condemn these people, because some of this is done electronically using computers," he said.