Sepp Blatter struck a "gentleman's agreement" with Michel Platini to pay him a million Swiss francs ($1.02 million) a year as an adviser, the former world football chief told their trial on Thursday.

The French football legend had jokingly asked Blatter for a million, without specifying the currency, and the then-FIFA president agreed, with part of the money -- outside of the contract they signed -- to be paid "later", Switzerland's Federal Criminal Court heard.

Blatter and Platini are being tried over a two-million-Swiss-franc payment in 2011 to the former France captain, who by that time was in charge of European football's governing body UEFA.

The trial in the southern city of Bellinzona -- following an investigation that began in 2015 and lasted six years -- opened on Wednesday.

Thursday's first day of evidence saw the trial hear directly from Blatter and Platini -- at points, to the court's astonishment -- about the casual nature behind the scenes at the pinnacle of world football.

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 Swiss francs, which was paid in full by FIFA.

Platini, 66, is regarded among world football's greatest-ever players. He won the Ballon d'Or, considered the most prestigious individual award, three times in the mid-1980s.

Blatter, now 86, joined FIFA in 1975 and became the president of world football's governing body in 1998.

Blatter told the court he turned to Platini for advice, which involved political trips, reforming the international calendar and helping the national federations financially.

"When I was elected as president of FIFA, we had a bad record. But I thought that a man who had been in football could help us -- FIFA and myself," Blatter told the court.

Platini said: "When Mr. Blatter asked me to be his adviser, he asked me what salary I wanted. I was surprised that he asked me this question and I said to him: 'I want a million'.

"Sepp told me: 'a million what?'. And jokingly, I said, 'pesetas, liras, rubles, marks; you decide'.

"He said to me: 'OK, a million Swiss francs' recalled Platini, adding that he had "succumbed to the charisma" of the Swiss former football administrator.

Josephine Contu Albrizio, heading the three judges hearing the trial, hesitated, then sought to make sure that Platini understood "the differences in values" between the currencies.

"I had never been in an administration like FIFA, I don't know how it works," said Platini. "I answered like that: a million".

Michel Platini is facing fraud charges alongside former FIFA boss Sepp Blatter
Michel Platini is facing fraud charges alongside former FIFA boss Sepp Blatter AFP / Fabrice COFFRINI

Blatter recalled that Platini "told me: 'I'm worth a million'. I told him: 'So you'll be with me for a million'," he said, adding that the Frenchman was "worth" the money.

The prosecution accuses Blatter of having signed off an invoice for two million Swiss francs presented to FIFA by Platini in 2011, almost nine years after the end of his work as Blatter's adviser.

Blatter said the annual contract for 300,000 Swiss francs was a temporary arrangement.

Platini "told me 'That's not everything', and I answered: 'The rest will come later'," the former FIFA chief said.

Blatter insisted that he had agreed with Platini a remainder to be settled when FIFA's fragile finances would allow it, in a "gentlemen's agreement" concluded orally and without witnesses.

"It's a salary that was due," he insisted.

"I don't know why we are in a criminal hearing for an administrative procedure," he said in a strained voice, repeating that since the investigation started in 2015 he had been through "seven years of punishment -- an eternity".

The judges were mystified as to why Platini had only invoiced FIFA in 2011 for two million francs, rather than the 2.8 million he would have been owed under the oral contract.

"It's my fault. I got it wrong," Platini said -- to laughter in the courtroom -- saying he had not checked how much he had already been paid.

"I realised this when the prosecutor showed me the 1999 contract during questioning," he said.

Judge Contu Albrizio asked Platini why he never bothered to mention the outstanding balance.

Platini said: "I never felt worried about the money, because since the age of 17, I have earned a good living."

Blatter and Platini are 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 trial will conclude on June 22, with the three judges expected to deliver their verdict on July 8.

If convicted, the pair could face up to five years in jail or a fine.