* 'Father-son' relationship descends into acrimony
* Blatter to influence race despite standing down
* Fans, sponsors look to next leader to clean up sport
By Simon Evans and Brian Homewood
ZURICH, Aug 20 (Reuters) - An increasingly bitter rift between FIFA president Sepp Blatter and UEFA chief Michel Platini looks set to define the campaign to elect the next leader of soccer's corruption-plagued world governing body.
The tense relationship between the Swiss and the Frenchman, once described by Blatter as being "like father and son", has descended in recent days into open conflict.
"Blatter will do whatever he can to stop Platini becoming president of FIFA," said a former senior official with the organisation. "He hasn't yet found the candidate he thinks can beat Platini, but you can be sure he is searching for one".
An acrimonious and divisive contest is the last thing FIFA needs as it picks a new leader to succeed Blatter and clean up a bribery scandal in which U.S. authorities have indicted nine soccer officials and five sports marketing executives.
U.S. and Swiss prosecutors are investigating corruption at the highest levels of the world's most popular sport, including in the awarding of the 2018 and 2022 World Cups to Russia and Qatar, in a saga that has dismayed fans and worried powerful sponsors like McDonald's Corp, Coca-Cola and Visa.
In a sign of the deepening rancour, Blatter said in an interview with Dutch newspaper Volkskrant on Saturday that there was an "anti-FIFA virus" at UEFA, the European governing body headed by former French star Platini.
He further alleged that Platini had intimidated his family before May's FIFA election, in which Blatter defied the growing corruption scandal to get himself returned for a fifth term -- only to announce four days later that he would step down.
Platini's camp dismissed the charge as "ridiculous".
In turn, UEFA said it had complained to FIFA's ethics officials about a dossier it suspects was distributed from FIFA headquarters in a "smear campaign" against the 60-year-old Frenchman.
Platini, one of the most elegant midfielders the game has seen, teamed up with Blatter in 1998 when he became his personal adviser at FIFA, a role he held for four years.
"Blatter loves to be around famous people, whether it be heads of state, politicians or players. While Platini was at his side, it worked perfectly for him, but when Platini gained power himself, that changed the dynamic," said another former FIFA official, who knows the 79-year-old well.
In 2007, Platini became president of UEFA, defeating an old rival of Blatter's, Lennart Johansson of Sweden. Gradually, as he imposed his own authority, clashes broke out with Blatter over policy issues and sources say the alliance became strained.
One source said the rift was partly to blame for the fact that UEFA and FIFA now use different refereeing systems for their competitions. FIFA has adopted goal-line technology while UEFA employs two extra assistant referees, one on each goal line, to help the referee make penalty area decisions. Platini once jibed that Blatter did not like the system "because it was not his idea".
When the corruption scandal erupted in May, the relationship reached breaking point. Platini took a belligerent tone, hinting at a possible UEFA breakaway, and demanded Blatter quit.
"Blatter was already furious with the open criticism, but after the very aggressive press conference from Platini, the day before the May congress, that was the end," said the source.
The sense of bitterness at the breakdown of their rapport, was evident in Blatter's comments in the Volkskrant interview.
"He has changed, I haven't," said Blatter, "You will have to ask him about his character. I don't know what goes on in his head".
What's in Platini's head at the moment is how to take Blatter's job, and whether the Swiss can conspire to stop him.
Certainly, there appears to be room in the election field for a candidate who could take advantage of lingering loyalty to Blatter among some national federations and defeat Platini, whose support does not look as solid as some observers expected.
When Platini launched his campaign last month, his team indicated he had support from four of the six regional football confederations, suggesting an easy march to power.
Weeks later, the picture is far less clear. The African and Oceania confederations and CONCACAF, which covers North and Central America and the Caribbean, have all yet to state who they will back, and collectively control 100 of the 209 votes.
While Platini's camp are confident of support from South America, Europe and Asia, the picture is complex. The head of the Asian confederation, Sheikh Salman Bin Ebrahim Al Khalifa of Bahrain, has said he will back the Frenchman, but it is by no means clear that all of Asia agrees.
While South Korea's former FIFA vice president Chung Mong-joon appears a rank outsider, Jordan's Prince Ali Bin Al Hussein is also considering running and could pose a bigger threat to Platini's support base in Asia.
Even within Platini's own confederation, support may not be unanimous, with Blatter still enjoying influence in Russia and other parts of Eastern Europe.
But all those calculations presume there will be a viable alternative to Platini.
So far, the field looks thin, with only South African Tokyo Sexwale, an apartheid-era political prisoner turned businessman who is weighing up a possible run, looking like the kind of candidate Blatter might back.
Those who know Blatter say the future of FIFA after he has gone weighs heavily on his mind.
"He is more worried about the change of FIFA president than he is about the FBI investigation," said another source.
"The two things he wanted at the end of his career were the Nobel Peace Prize and honorary president of FIFA. He has lost any chance at the first, of course, and the second looks less likely too."
Whoever wins, Blatter, who joined FIFA as a development officer in 1975 and often spends his weekends working at its bunker-like headquarters, may find the transition to retirement a difficult one, said the source.
"It is going to be extremely difficult for him to adjust. He knows lots of people but he has no real friends." (Editing by Mark Trevelyan)