A Queens prosecutor has resigned after being demoted by the DA for misleading a judge about the whereabouts of a key witness in a murder case, authorities said yesterday.

Claude Stuart quit on Wednesday, after a judge ordered a new trial for Tyrone Johnson last month, a spokesman for the District Attorney Richard Brown said.

Johnson’s murder conviction was tossed out after Brown admitted Stuart wrongly told a judge and a defense lawyer that he couldn’t find the witness – who says Johnson wasn’t involved in the murder.

Stuart had spoken to the witness four days before the trial began, prosecutors said.