In a potential blow to the prosecution, a prison informant testified Thursday — at the trial of a man charged with the murder of 6-year-old Etan Patz — that a prior suspect in the case confessed to abducting and abusing the child the day he vanished in 1979.

Jeffrey Rothschild, a reformed career conman, told jurors he shared a cell with Jose Ramos in an Orange County prison in 1991.

After weeks of gaining his trust, the convicted pedophile showed him on a hand-drawn map where he’d picked up Patz near his SoHo home.

Ramos said he’d taken the child to his East 4th Street apartment and sodomized him.

The depraved child molester told him that the boy was dead but stopped short of admitting he’d killed him, the defense witness told jurors at the Manhattan murder trial of Pedro Hernandez.

“That bastard knows I did it,” Ramos told him of a federal prosecutor who was trying to build a case against him for the boy’s murder, Rothschild said.

“‘He can’t prove it and it’s killing him. Good — Let it kill him. Let it kill him. We’ll both be buried next to Etan.’

“And I said, ‘Is the boy dead?’ And he said, ‘What the f—k do you think? Of course he’s dead, but there’s no proof.’”

The testimony bolstered Hernandez’s defense that Ramos is the more likely suspect in the tot’s death.

Hernandez, a former SoHo bodega clerk, confessed to the crime in 2012. But his lawyers insist the admission was false and coaxed from a man with a history of mental illness.

Patz disappeared from a SoHo street the first day he walked alone to the school bus stop nearly 35 years ago. His body was never found.