Ex-WorldCom chief Bernard Ebbers' (search) fraud and conspiracy convictions should be overturned because his trial was "fundamentally flawed," his lawyers argued in papers filed with a federal appeals court Thursday.

His lawyers urged the the Second Circuit U.S. Court of Appeals to set aside the guilty verdicts, saying a lower court should have forced the government to grant immunity to several prospective defense witnesses and also made two other significant mistakes.

Ebbers, 64, was found guilty in March of orchestrating an $11 billion accounting fraud that drove WorldCom (search) into bankruptcy. He was sentenced to 25 years in prison, but remains free on bail while he pursues an appeal.

In the 96-page brief, his lawyers said the court wrongly instructed the jury that it could convict Ebbers on the basis of so-called "conscious avoidance" of knowledge of the fraud at WorldCom.

His lawyers also said that while all of the charges were "directly predicated on allegedly fraudulent accounting entries in WorldCom's financial statements," the jury was improperly permitted to vote for a conviction even if the telecommunications company's underlying accounting entries fully complied with applicable accounting standards.

"These errors greatly hindered Ebbers' ability to present a defense, unfairly lowered the government's burden of proof and offered the jury a compromise path to conviction," according to the brief from his legal team, led by defense attorney Reid Weingarten of Steptoe & Johnson LLP in Washington.

"It is likely that the outcome of this trial would have been different if even one of these errors had been corrected," they wrote.

His lawyers also called his 25-year prison term "unreasonable and legally erroneous."

They said Ebbers' sentence was five times as long as that given to ex-WorldCom financial chief Scott Sullivan (search), who pleaded guilty to fraud charges and was the government's star witness at Ebbers' trial.

Sullivan, however, was termed the "architect" of the fraud by Manhattan federal judge Barbara Jones, who oversaw the criminal case, Ebbers' lawyers wrote.

Ebbers was convicted of nine counts of fraud, conspiracy and filing false documents with securities regulators. A federal jury deliberated for eight days in his case.

The government's response brief to Ebbers' appeal is due by Oct. 28, according to prosecutors.