NEW YORK - Disgraced Wall Street mogul Bernard Madoff will be sentenced as planned on June 29, but prosecutors have asked the judge to delay by 90 days a decision on restitution he is to pay to his victims.

In a memorandum sent to US District Judge Denny Chin, the prosecutors said that of between US$50 billion and US$60 billion dollars swindled by Madoff from investors, only US$13 billion have been traced.

"For the reasons set forth above, the government respectfully submits that sentencing on June 29 should proceed, and that the court should order a ninety day extension of the time in which the court will order restitution or find that restitution is impracticable," the memorandum said.

Chin could sentence Madoff, 71, to up to 150 years in prison. The former chairman of the Nasdaq stock exchange operated the fraud for decades, conning investors into depositing billions of dollars that were then used to pay fictitious returns.

Madoff has been in jail since March after admitting guilt in the scheme, the biggest in Wall Street history and estimated to have involved around 61 billion dollars.

Many of the investors were elderly retirees who thought their life savings were in safe hands.