Hayes and another suspect, Joshua Komisarjevsky, 20, of Cheshire, have pleaded not guilty to capital felony murder, sexual assault and other crimes in the deaths of Jennifer Hawke-Petit and her daughters, 11-year-old Michaela and 17-year-old Hayley.
Hayes and Komisarjevsky had previously offered to plead guilty in exchange for life in prison without parole. Defense attorneys have said prosecutors rejected the offer and want the death penalty, but prosecutors have refused to comment.
Hayes, 46, could be executed if he is allowed to change his plea to guilty. The judge told him either a jury or a three-judge court would determine his sentence, because he is not allowed to plead guilty to a death sentence.
"This is potentially a momentous development in the case," the judge said, adding all sides need some time to consider how to proceed.
A change of plea would have to be accepted at a formal hearing.
The plea would cover all charges against him.
Blue said "certain indications" in the competency report led him to ask Hayes whether he wanted to change his plea.
"I wish to change," Hayes responded, confirming after further questions from the judge that he wanted to change his plea to guilty and that it was a firm decision.
Prosecutor Michael Dearington had no immediate reaction to Hayes' saying he wanted to change his plea.
Hayes' attorneys would not comment outside of court on why they believe his change of plea should not be accepted. They are expected to outline their arguments in court papers by Monday, in time for a hearing on Tuesday. They did confirm that Hayes' move was not part of a plea deal with prosecutors.
Telephone messages were left with Komisarjevsky's attorney.
Hayes and Komisarjevsky were accused of breaking into the Petit home, beating Dr. William Petit and holding the family hostage for hours before strangling his wife and setting the house on fire. The girls, who had been tied to their beds, died of smoke inhalation.
Jury selection has been ongoing in preparation for Hayes' trial, which had been scheduled to begin in September.
Rick Healey, an attorney who is spokesman for Dr. Petit, said he and Petit had no comment. Petit, who has expressed frustrations over delays in jury selection, was not in court Thursday.