Renzi, who made the comments in a television interview with state broadcaster RAI, has repeatedly pushed back the date of the referendum. Early this year he said it would probably be held in early October.
The reform, which has been approved by parliament, aims to reduce the role of the upper house Senate and rein in the powers of regional governments. Opinion polls suggest the outcome is currently too close to call.
Renzi has repeatedly said he would resign as prime minister and quit politics if the reform is rejected in the referendum.
He said in September 6's interview that he had not changed his mind about this, but no longer wished to discuss his own future.