ZURICH—Soccer’s world governing body issued eight-year bans for alleged ethical violations to two of the most powerful men in the game: FIFA President Sepp Blatter and his former heir-apparent Michel Platini.

The move, announced on Monday, made the pair the highest-profile casualties of the continuing world-wide investigations into corruption and ethical wrongdoing at FIFA. Both men denied any impropriety and said they would appeal. Mr. Platini, a suspended member of FIFA’s ruling executive committee, said he planned to take his case to the FIFA Appeals Committee and the Court of Arbitration for Sport, an independent panel with final say in sporting disputes.

Mr. Platini said he still hoped to seek the FIFA presidency in a special election on Feb. 26. But FIFA insiders say the chances of him winning an appeal before that date are nil. Only three months ago, the 60-year-old Mr. Platini had been the front-runner. Now that distinction belongs to Sheik Salman Bin Ebrahim Al Khalifa, a FIFA executive from Bahrain, according to FIFA insiders.

The eight-year suspensions come as both men already were serving 90-day bans. That first suspension automatically removed Mr. Platini as head of UEFA, the governing body of European soccer, which is home to the game’s richest clubs and players. On Monday, the organization issued a statement saying, “Naturally UEFA is extremely disappointed with this decision, which nevertheless is subject to appeal. Once again, UEFA supports Michel Platini’s right to a due process and the opportunity to clear his name.”

The central issue is what Swiss authorities called a “disloyal payment” of 2 million Swiss francs ($2.02 million) from FIFA to Mr. Platini in 2011. Mr. Platini has acknowledged receiving the payment but denies any impropriety. He has said that the funds were his salary for advisory work he carried out at FIFA between 1998 and 2002 at the behest of Mr. Blatter—on a handshake.