Uli Hoeness, president of European football champions Bayern Munich, has admitted in court to defrauding Germany’s tax authorities of 18m euros (£15m; $25m).
Prosecutors had earlier accused him of evading a far smaller sum of 3.5m euros in taxes and are seeking a jail term.
The former World Cup-winning German international footballer, 62, kept the funds in a secret Swiss bank account.
He told the court he deeply regretted “my wrongdoing”.
He is Germany’s Mr Football. Not only was Uli Hoeness a star on the field, he became a great mover and shaker off it. As president of Bayern Munich, one of the world’s greatest clubs, he was the man with whom everybody from Chancellor Angela Merkel down was pleased to be seen. This morning, in a court room in Munich, the allure and the glamour seemed a long way away.
He is accused of not declaring more than 33m euros in income on which he should have paid 3.5m in tax. But his lawyer has now said that this figure of unpaid tax is actually higher.
Mr Hoeness came forward to admit the account only after the media got wind of it. He received a phone call warning him of the investigation. He received this phone call when he was having lunch with Chancellor Merkel, a measure of the exalted circles in which he moved.
In court, he stood in the darkest of suits smiling politely – and perhaps nervously – to the lawyers in their robes around him.
At 62, he is portly, perhaps twice the weight he was when, as a magical forward in 1974, he helped Bayern Munich win the European Cup and Germany the World Cup.
“I will do everything necessary to ensure that this depressing chapter for me is closed,” he said.
The former Germany forward, who helped the national team win the 1972 European Championship and then the World Cup two years later, came clean about his secret bank account last year, filing an amended tax return in the hope of an amnesty in return for paying the tax he owed.
But prosecutors say he did so because investigators were already on his case.
The penalty for tax evasion can be 10 years in jail, though the prosecution says it will seek a seven-year sentence. A verdict is expected on Thursday.
Munich state prosecutor Achim von Engel read out the indictment against Mr Hoeness shortly after the start of the trial, described as one of the most spectacular of the year by the German newspaper Sueddeutsche Zeitung.
He alleged that the defendant had failed to declare the income he held at Vontobel bank in Switzerland.
Giving evidence later, Mr Hoeness said he had used the money for large-scale gambling on the foreign currency markets, losing far more than he ever gained.
“Between 2002 and 2006 I really gambled with sums that today I find difficult to grasp. For me it was a kick; pure adrenalin,” he said.
It was while he was having lunch with Chancellor Angela Merkel on 15 January in Berlin last year that he was made aware of press interest in his finances, he told the court. Vontobel phoned him, he said. warning him that journalists from Stern magazine were making inquiries.
Before the scandal emerged, he was considered to be on good terms with Mrs Merkel, who has since said she is disappointed with him.
Mr Hoeness said he had already decided to come clean about his taxes to the authorities.
He is charged with not declaring 33,526,614 euros in tax returns from 2004 to 2009 in income on which he should have paid 3,545,939 euros and 70 cents.
Despite the tax evasion scandal, Mr Hoeness remains a very popular figure at the club he helped build up.
He offered his resignation at last year’s annual meeting but was backed by the supporters and the club’s board.
Bayern Munich refused to comment before the trial but said its chairman and former star Karl-Heinz Rummenigge would speak after the verdict.
An added potential difficulty for Mr Hoeness, our correspondent reports, is that the judge in the case, Rupert Heindl, has been dubbed “judge merciless” in the German press, after jailing a pensioner in his 70s.
He is thought not to like deals whereby transgressors avoid jail by repaying what they have gained illegally.
Bayern Munich are facing an important tie against Arsenal on Tuesday in the last 16 of the European Champions League. Coach Pep Guardiola said: “We can help him most with a win over Arsenal.”