Former FIFA president Blatter, 86, and Platini, 66, start a two-week trial at Switzerland's Federal Criminal Court in the southern city of Bellinzona, following a mammoth investigation that began in 2015 and lasted six years.

The pair are being tried over a two million Swiss franc ($2.08 million) payment in 2011 to Platini, who was then in charge of European football's governing body UEFA.

They are accused of having, to the detriment of FIFA, illegally obtained the payment, plus social security contributions of 229,126 francs, in favour of Platini.

The former French football great "submitted to FIFA in 2011 an allegedly fictitious invoice for a (alleged) debt still existing for his activity as an adviser for FIFA in the years 1998 to 2002," according to the court.

The defendants are both accused of fraud and forgery of a document. Blatter is accused of misappropriation and criminal mismanagement, while Platini is accused of participating in those offences.

The indictment was filed by the Office of the Attorney General of Switzerland (OAG).

The trial will conclude on June 22, with the three judges delivering their verdict on July 8.

The defendants could face up to five years' imprisonment or a fine.

Both FIFA and UEFA are headquartered in Switzerland, in Zurich and Nyon respectively.

'Oral contract'

Platini and retired Swiss football administrator Blatter were banned from the sport at the very moment when Platini seemed ideally-placed to succeed Blatter at the helm of world football's governing body.

The two allies became rivals as Platini grew impatient to take over, while Blatter's tenure was brought to a swift end by a separate 2015 FIFA corruption scandal investigated by the US Federal Bureau of Investigation.

In the Bellinzona trial, the defence and the prosecution agree on one point: Platini was employed as an adviser to Blatter between 1998 and 2002. They signed a contract in 1999 for an annual remuneration of 300,000 francs.

"The compensation agreed in accordance with this contract was invoiced by Platini on each occasion and paid in full by FIFA," said the OAG.

However, more than eight years after the end of his advisory role, the former France captain "demanded a payment in the amount of two million francs", the OAG alleged.

"With Blatter's involvement, FIFA made a payment to Platini in said amount at the beginning of 2011. The evidence gathered by the OAG has corroborated that this payment to Platini was made without a legal basis. This payment damaged FIFA's assets and unlawfully enriched Platini," the federal prosecution alleges.

The men insist that they had, from the outset, orally agreed to an annual salary of one million francs.

"It is outstanding salary, owed by FIFA, under oral contract and paid under conditions of the most perfect legality. Nothing else! I acted, as in all my life and career, with the utmost honesty," Platini said in a statement sent to AFP.

As a civil party, FIFA wants to be reimbursed the money paid in 2011 so that it is "returned to the one and only purpose for which it was intended: football", its lawyer Catherine Hohl-Chirazi told AFP.

Ballon d'Or winner

Joseph "Sepp" Blatter joined FIFA in 1975, became its general secretary in 1981 and the president of world football's governing body in 1998.

He was forced to stand down in 2015 and was banned by FIFA for eight years, later reduced to six, over ethics breaches for authorising the payment to Platini, allegedly made in his own interests rather than FIFA's.

Platini is regarded among world football's greatest-ever players. He won the Ballon d'Or, considered the most prestigious individual award, three times -- in 1983, 1984 and 1985.

Only Lionel Messi (seven) and Cristiano Ronaldo (five) have won more Ballons d'Or than Platini.

Platini was UEFA's president from January 2007 to December 2015.

Platini appealed against his initial eight-year suspension at the Court of Arbitration for Sport, which reduced it to four years.