Detectives investigating the disappearance of Madeleine McCann are reportedly searching for a waitress in Bulgaria who may hold the key to the mystery.

Two witnesses claimed to have seen a woman dressed in purple standing outside the McCann family’s holiday apartment on the night the three-year-old went missing ten years ago.

The woman has been named as Luisa Todorov, 58, and she and her husband Stefan, 50, are both said to have worked at the Ocean Club resort in Praia da Luz in 2007, the Express reported.

The newspaper adds that Scotland Yard officers are in Bulgaria trying to track the couple down in order to re-interview them as significant potential witnesses.

The pair gave statements to Portuguese police five days after the abduction and it is believed they may have since travelled to Bulgaria looking for work. They have denied any involvement with the case.

In May Jenny Murat told The Sun she had seen a woman dressed in purple staring intently in the direction of the apartment Madeleine disappeared from.

She said: “I don’t know who she was but she caught my eye because she was dressed in purple-plum clothes.

“It struck me as strange because it’s so unusual for anyone, particularly a woman, to be standing alone in our resort, just watching a building.”

Murat reported her sighting to the police the next day, after learning a child had gone missing from the resort. Another witness reported having seen a woman of the same description half an hour later.

Spanish criminologist Heriberto Janosch Gonzalez told Portuguese news channel Correio da Manha: “It seem to me that it is very likely that the purple woman the police are looking for is Mrs Todorov. They are the only ones with links to Bulgaria and this explains why Scotland Yard was there.”

In September the Home Office agreed to give Scotland Yard another £154,000 to extend its search for the missing girl until the end of March.

It is believed the investigation – named Operation Grange – has already cost £12million.

Madeleine was nearly four when she vanished in May 3, 2007 from her family’s holiday apartment as her parents dined with friends at a tapas bar nearby.

Her parents, Kate and Gerry McCann, of Rothley, Leicestershire, have often spoken of their bitter regret about leaving her and then two-year-old twins Sean and Amelie alone. Their daughter would now be a teenager and the family have never given up the search and remain hopeful she is alive.

A Scotland Yard spokesman said the force was not prepared to give a running commentary on the investigation