Madeleine McCann Older
An age progression image shows what Madeleine might look like at age 9.

Police in London investigating the disappearance of Madeleine McCann have been provided with additional money to track down a "critical witness" -- a woman missing from Portugal -- in the case. Authorities believe that the woman, believed to be East European, could hold key clues.

Sources told the Sun that the woman lived near the holiday flat in Praia da Luz on the Algarve from where Madeleine, then aged three, vanished in May 2007. But the woman is believed to have left the country after her husband’s death. An additional $201,274 has been added to the $15 million Operation Grange to find Madeleine.

“She may have seen or heard something which to her may seem insignificant but to us could prove vital. If she recognizes herself she should contact police. She has nothing to fear. This is just to eliminate her from inquiries," the source, close to the operation said, describing the woman as a "critical line of inquiry."

The 3-year-old went missing after her parents left the girl and her 2-year-old twin siblings alone inside a vacation apartment rental in Portugal in 2007 while they went out to dinner. Kate and Gerry McCann have adamantly maintained their innocence in relation to their daughter’s disappearance. They were named as official suspects four months after her disappearance, but Portuguese police dropped the case in July 2008 due to lack of evidence. Authorities at Scotland Yard reopened the case in 2013. Four people named as suspects were also cleared in 2013. The operation to look for Madeleine, called Operation Grange, was launched in May 2011.

Police still believe that Madeleine may have been taken during a burglary that had "gone wrong."

Her parents Kate and Gerry insist they must continue searching for their daughter as there is "absolutely nothing" to suggest their daughter has been harmed.

Speaking a few weeks ago, a spokesman for the family said the McCanns remain "extremely thankful" to police for requesting extra funding.

Their representative, Clarence Mitchell, said: "They are very encouraged there remains work to be done that requires them seeking an extra budget."

"They are grateful to all those officers who are still working on the case and are actively looking for Madeleine," he said. "They appreciate everything the police have done and are going to get a resolution after all this time."

It remains unclear whether Madeleine was alive or dead. Ahead of the 10 year anniversary of her disappearance in May, investigators released a statement regarding the situation, clarifying that it was still technically a missing person case.

“It is a missing person’s inquiry,” Metropolitan Police Assistant Commissioner Mark Rowley said. “But as a team, we are realistic about what we might be dealing with — especially as months turn to years.”

The initial Portuguese investigation into Madeleine's disappearance saw a series of mistakes that were made in the early hours of the probe.