Convicted fraudster Tom Petters asked a court Friday to throw out his 50-year prison sentence, after claiming that his attorney in 2008 failed to tell him that U.S. prosecutors had once offered him a 30-year deal in exchange for a guilty plea.

In his complaint filed ­Friday, Petters, 55, said the court should regard his sentence as “unlawful and unconstitutional” and that his conviction should be vacated. He asked that new plea and sentencing hearings be conducted.

In an affidavit filed in U.S. District Court, Petters said he learned of the 30-year plea offer only after his April 2010 conviction for his role in the $3.65 billion Ponzi scheme.

At that time, Petters said he learned that Assistant U.S. Attorney John Marti had spoken to his attorney, Jon Hopeman, twice in October 2008 and offered the plea deal. The offer was never conveyed to him, he said.

Instead Hopeman met with Petters in December 2008 and allegedly told him that he “had received no plea offer from the government,” court documents said. Later that month, Hopeman allegedly met with Marti in person and refused the plea offer, saying that “As a matter of personal pride, I did not believe that I could advise Mr. Petters to plead guilty to a 30 year cap. … This suggested that 30 years was an appropriate sentence. … My professional integrity would not allow him to do so.”

Petters was convicted and sentenced to 50 years in prison. He is serving his time in the Leavenworth penitentiary in Kansas.

In his affidavit Friday, Petters said: “If Mr. Hopeman had informed me of the government’s offer in a timely fashion and before trial, I would have accepted the offer and pleaded guilty.”