Jeffrey Skilling, the former chief executive of Enron, has begun serving his 24-year prison sentence for fraud after an appeals court in the US denied his request for bail.
On Monday, the court said Skilling could remain free while it considered bail as he appealed against his convictions, but it decided not to grant bail late on Tuesday.
Judge Patrick Higginbotham of the fifth circuit court of appeals wrote in his order that "Skilling raises no substantial question that is likely to result in the reversal of his convictions on all of the charged counts".
Skilling's lawyer was not immediately available for comment.
According to the Federal Bureau of Prisons website, Skilling was headed to a federal prison in Minnesota on Tuesday.
In May 53-year-old Skilling was found guilty on 19 charges of conspiracy, fraud and insider-trading for his role in hiding Enron's financial condition from investors while the energy company struggled before it collapsed in 2001.
Skilling's sentence is the longest handed out to any of the former Enron executives found guilty.
Kenneth Lay, who founded the company, also was convicted of conspiracy, fraud and insider trading charges, but he died of heart disease on July 5.
The low-security prison in Waseca, Minnesota, is 121km south of Minneapolis, the state capital.