Private detectives hired by Madeleine McCann's parents today released an e-fit of a "Victoria Beckham lookalike" they believe may have information about the missing girl.
The woman was seen by two British men in Barcelona in Spain three days after the child went missing from Praia da Luz in Portugal.
She had a brief conversation with one of the men which suggested she might know something about what happened to Madeleine, the McCanns' investigators said.
Former detective inspector Dave Edgar, who is leading the McCanns' investigation, said it was possible that Madeleine could have been taken to Spain by sea from where she disappeared in the Algarve.
He told a press conference in central London: "Madeleine most definitely could have got from the area to Barcelona by yacht."
The British men, who were part of a larger group, saw the woman in the early hours of May 7 2007 while on a night out in Barcelona's popular Port Olimpic Marina.
The woman was well-dressed but appeared agitated and kept walking up and down outside the El Rey de la Gamba restaurant bar as if she was waiting to meet somebody, the witnesses recalled.
When one of the British men approached and spoke to her, she said something that the McCanns' team described as "potentially significant to the investigation".
Mr Edgar refused to give any details about this conversation, saying only: "She's a significant individual, I wouldn't describe her as a suspect."
After speaking to the witness, the woman - who is described as having an Australian accent - went into a bar next door and sat down on her own.
While in the bar she had a heated conversation with a local in what seemed to be fluent Spanish.
The main witness, who is a married 41-year-old professional without children, only contacted the McCanns' investigation team about six weeks ago.
Mr Edgar said that his delay in coming forward was for "personal reasons".
"We are satisfied that he is a very credible witness and the account he's given is clear and concise," he said.
The woman was last seen walking away towards a local hospital and more distant ferry and cruise ship terminals.
The main witness described her as aged 30 to 35, slim, about 5ft 2ins tall and with short brown hair.
She was wearing expensive-looking blue jeans and a jersey top, possibly in a pastel colour.
He also described her as "a bit of a Victoria Beckham lookalike".
Mr Edgar said the witness had described the e-fit - which was produced by a British police artist - as "80 per cent" accurate.
He said the man had been drinking that night but was not drunk, and the woman was sober.
The McCanns' private detectives travelled to the marina in Barcelona in search of extra information, but all CCTV footage was wiped a long time ago.
They have informed Portugal's Policia Judiciaria - who led the investigation into Madeleine's disappearance until it was shelved last year - about the new lead.
They will now liaise with the Australian Federal Police and the Spanish authorities.