The woman said she reported it at the time in May, 2009 – but was only contacted by investigators when she re-reported the potential lead two months ago.
"I saw this little blonde girl looking just like Maddie. I've never felt so strongly about something," the woman, a mother herself, said.
The Sun reports that Madeleine was spotted having lunch in Montgomery County, Maryland, with a blonde woman and two men, one of whom the witness described as ' grey-haired and distinguished', the other 'scruffy looking with a foreign accent'.
The woman, who has requested anonymity, said the little girl approached her own daughter, but was 'pulled back' by the woman she was with before she had opportunity to speak.
Madeleine went missing from her family's holiday flat in Praia da Luz, Portugal, in 2007 when she was three years old.