LONDON - Detectives are investigating whether Madeleine McCann, the toddler who vanished in Portugal seven years ago, may have been abducted by a serial sex attacker.

One of the suspect's victims was a 10-year-old girl who was assaulted in 2005 in the same resort where Madeleine went missing.

Scotland Yard has recently learned for the first time of five sexual assaults, and one attempted sexual assault, on British girls between 2004 and 2006. Police say they attach particular significance to one of the attacks: the sexual assault in 2005 of a British 10-year-old girl in Praia de Luz, the resort where Madeleine's family were staying. That attack was not reported at the time.

McCann was 3 when she vanished from the family's resort room in 2007 while her parents dined nearby.

Madeleine McCann, who went missing in May 2007, is shown in this undated photo. PA/AP Photo

These latest cases were brought to police attention after a public appeal for information last month.

Senior investigating officer Andy Redwood calls the new leads "extremely positive" and a "priority" for his investigation. Scotland Yard have requested case files for the six attacks from Portuguese police.

The British investigation is now examining 18 cases where a male intruder entered the apartments and villas of British vacationers in the Western Algarve region of Portugal. They include nine sexual assaults and three "near-misses," officials said. The victims were all British girls aged between 6 and 12 years old. Fourteen of the attacks were reported to the police at the time.

Detective Chief Inspector Redwood says he is confident that some if not all of the cases are linked because of their "significant similarities."

In order to protect the identities of the victims, police are not disclosing why some of the attacks were not reported earlier. While some of the attacks were known to police in Portugal, they were not considered relevant by them to the McCann case because they didn't involve abduction.

Deputy Assistant Commissioner Martin Hewitt says he is "cautiously optimistic" that police in Portugal will soon move forward with Scotland Yard's requests for help.

Senior British detectives have expressed frustration with the slow pace of the judicial process, and the cooperation they have received from their Portuguese counterparts.

Portuguese authorities have resisted British requests to form a joint investigation team.

In March police appealed for help regarding 12 cases of a male intruder entering vacation homes between 2004 and 2010. The man sexually assaulted five British girls aged between 7 and 10 in their beds.

They said that witnesses described the suspect as tanned, with short untidy hair. He spoke in English with a foreign accident, and smelled strongly.

Scotland Yard has still not established the identity of a man seen by three witnesses carrying a child fitting Madeleine's description on the night that she disappeared.