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Private investigators hired by Madeleine's parents Gerry and Kate McCann were alerted to the woman by a new witness, a British man who had a brief conversation with her outside a popular waterfront bar in the Spanish city of Barcelona just three days after the youngster vanished from a Portuguese holiday resort in May 2007.
While investigators do not believe the woman, thought to be in her 30s, is a suspect in the case, they do believe what she told the British man could be "potentially significant" to their investigation.
A sketch of the woman, based on the British man's description, was released in London as investigators launched an appeal for the woman or anyone who thinks they might know her to come forward.
The former policeman who is heading the McCanns' search for their daughter, Dave Edgar, said while he did not want to reveal details of the conversation the woman had with the British man in the area of the El Ray De La Gamba bar at the Port Olimpic Marina, he believed she had an Australian accent.
"It's a strong lead. It was a significant conversation," Edgar told reporters.
"We obviously want to track this individual and speak to them.
"She is a significant individual. I wouldn't describe her as a suspect." The woman is described as being aged between 30 and 35, of slim build and "glamourous" appearance, with short brown hair and about five foot, two inches tall. She was wearing expensive looking blue jeans and possibly a pastel coloured jersey top.
The witness described her as a "Victoria Beckham look-alike".
Edgar and his fellow investigators will be contacting the Australian Federal Police as part of their search for the woman.
The British man, who was in Barcelona partying with friends, spoke to the woman around 2am on May 7 after noticing that she appeared agitated.
The pair spoke on the roadside outside the bar briefly before she walked off.
He told investigators he strongly believed the woman had an Australian accent, however a friend who also heard their conversation thought there was a possibility she could be from New Zealand.
Asked why it had taken the British man two years to come forward, Edgar said the man had explained it had been because of "personal reasons" that he did not want revealed.
However, investigators believed him to be credible and trusted the information he had provided.
"We are taking the witness at face value. He is a credible witness, a professional," Edgar said.
"I think he recognises he perhaps should have come forward earlier.
"He spent a little time with her so he was able to give us a good description of her." The man came forward about six weeks ago, prompting Edgar and his team to visit Barcelona to try to find anyone who might know the woman and check CCTV footage.
However, given that two years had gone by since four-year-old Madeleine disappeared from her family's rented holiday apartment in Praia da Luz on May 3, 2007, no CCTV footage was left and the bar staff had completely changed.
Edgar noted that it was possible to reach Barcelona by yacht in about five days from Portugal and even quicker by road or ferry.
"She could have definitely got from the area to Barcelona by yacht. It's entirely possible," he said.
A spokesman for the McCann family, Clarence Mitchell, said Madeleine's family were trying not to get their hopes up based on the new information.
"They are now just obviously waiting to hear and praying that this could be the piece in the jigsaw that could help them find Madeleine," he said.
"Sad to say we have had potential sightings of Madeleine and tip-offs, well meaning and malicious, from around the world. Madeleine has been allegedly seen in South America, Australia in the past, all over Europe.
"As a result, Kate and Gerry neither get their hopes up or down.
"I think they are obviously hoping that this will prove to be as significant as Dave believes it might be. But they wait to hear."
Anyone with information about the woman or Madeleine's disappearance can contact +44 845 838 4699 or investigationfindmadeleine.com.