Scotland Yard detectives investigating the disappearance of Madeleine McCann are planning to trace virtually every person in the Algarve resort where she vanished using mobile phone data.
British officers have asked police in 31 different countries to help them identify the owners of every mobile phone logged in Praia da Luz in southern Portugal in May 2007.
Holidaymakers from across the world could receive a call from the officers to ask them what they were doing there at the time.
The Metropolitan Police team investigating Madeleine's disappearance revealed yesterday that they are carrying out the first detailed trawl through vast amounts of mobile phone data in search of suspects.
They have already singled out 41 "persons of interest" in the case. This includes 15 British nationals, at least three of them newly identified since July.
Madeleine's parents, Gerry and Kate McCann, will make a new appeal for help on the BBC's 'Crimewatch' programme later this month in the light of "fresh, substantive" information uncovered by the Scotland Yard inquiry.
Over the summer, the Met detectives sent "international letters of request" asking for assistance from foreign police forces to Portugal and 30 other countries, most of them in Europe.
The requests relate to persons of interest and unidentified mobile phone numbers that were recorded in Praia da Luz around the time that Madeleine, then three, went missing between 9pm and 10pm on May 3, 2007.
Detective Chief Inspector Andy Redwood, who is leading the new inquiry, said that the phone data "could be" the key to solving the crime. But he stressed: "A lot of the focus of that is not necessarily to try to find the suspect, it's trying to find witnesses."
The data was available to the Portuguese detectives who first investigated Madeleine's disappearance but has not been analysed in detail until now.
The information includes the numbers of thousands of mobile phones that were switched on in Praia da Luz that day (© Daily Telegraph London)