Prosecutors are trying to prevent Raj Rajaratnam's defence team from trying to discredit Goldman Sachs Group chief executive Lloyd Blankfein as a trial witness with questions about government investigations into the bank.

The government has subpoenaed Blankfein and may call him as a witness as early as this week, according to letters written to prosecutors and a judge by lawyers for the Galleon Group founder in his insider-trading trial. The defence team has argued to the judge overseeing the case that it should be able to probe Blankfein about whether he is testifying "in the hopes of obtaining a better resolution" in ongoing government investigations.