Carlos Moreira, 65, told Britain's Sun that he saw a blonde girl asleep in a white van near Lisbon, Portugal, a day after McCann went missing on May 3, 2007.
"She was wearing a two-piece pajama set, pink and white or yellow," he told the newspaper, adding that she was "lying on a pile of clothes in the back."
"She moved one of her fingers but she was deeply asleep," he said. "When the van door was opened, she did not wake up as if she was drugged."
A source close to the couple tells PEOPLE that the sighting is "potentially significant" and may turn out to be more important than other clues, which have proven to be false hopes.
Clarence Mitchell, a spokesperson for Gerry and Kate McCann, tells PEOPLE, "Yes, we are aware of Mr. Moreira's account. People from the private investigation have spoken with him and are continuing to liaise with him. We will not go into further detail for operational reasons. But he has done the right thing in coming forward."
Moreira reportedly waited three years to come forward with this information because he dismissed the sighting at the time, believing police who claimed that all roads from the Praia da Luz resort had been closed.