"The adjudicatory chamber of the independent Ethics Committee ... Has notified Mr Joseph S. Blatter and Mr Michel Platini of the grounds for the decisions passed in December 2015," the body said in a statement.
In December, it banned the FIFA president and vice president from football for eight years, saying they had abused their positions over a $2 million payment made to Platini in 2011 for work carried out between 1999 and 2002.
At the time of the verdict, which heightened further the crisis rocking football's scandal-plagued world body, the court insisted there was "no legal basis" for the payment that Blatter authorised for Platini in 2011.
The tribunal did not provide further details Saturday of the reasoning behind its decision.
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Instead it stressed it had now "fulfilled its commitment to provide the grounds for the respective decisions to Mr Blatter and Mr Platini within the first half of January 2016."
At the time of the verdict, both men angrily vowed to
fight the bans, which started immediately.
The tribunal decision promises to end 79-year-old Blatter's four decades with FIFA in disgrace.
It also dealt a devastating blow to 60-year-old Platini's hopes of taking over as head of FIFA in an election on February 26.
The UEFA president pulled out of the race earlier this week, saying the ban has made it impossible for him to put together a campaign to take on the sport's most powerful job.
"In withdrawing, I am dedicating myself to my defence."
FIFA has since last May be rocked to its core by a cascade of corruption charges and arrests that culminated with the implication of the two long considered world football's most powerful men.
The US justice department has charged 39 individuals and two companies over graft within world football going back decades, in a sweeping prosecution that has sparked an unprecedented crisis at FIFA.