Martin A. Siegel’s sentencing was delayed again.

It was the third delay in the sentencing of the former investment banker who pleaded guilty to criminal charges arising from Wall Street’s insider trading scandal. Siegel is an important cooperative witness in the government investigation. U.S. District Judge Robert J. Ward was to have sentenced Siegel Thursday, a date rescheduled from June 9. The original sentencing date was to have been April 20. Charles M. Carberry, assistant U.S. attorney for securities and commodities fraud, said the sentencing date has been postponed again with no new time set. Neither Carberry nor Jed Rakoff, Siegel’s attorney, gave a reason for the postponement.