A federal appeals court in the US has denied bail to the former CEO of Enron Corp and ordered him to report to prison immediately to serve a 24-years prison sentence for securities fraud and conspiracy.

Jeffrey Skilling was on Tuesday denied his last minute plea to remain free on bail while he challenges his conviction.

"Skilling raises no substantial question that is likely to result in the reversal of his convictions on all of the charged counts," the court said.

Skilling maintains his innocence and said he was made a scapegoat. Defense lawyer Daniel Petrocelli said the formal appeal of his client's conviction would be filed in February or March. Skilling, 53, is to serve his term at the low-security Federal Correctional Institution in Waseca, Minnisota.

Petrocelli said Skilling will "likely" enter prison on Wednesday.

More than 5,000 jobs and $1 billion in employee pensions vanished when Enron collapsed five years ago, following revelations of a widespread accounting fraud allegedly directed by Skilling and former Enron chairman Kenneth Lay. Investors are suing to recover more than $30 billion in losses.