Detectives investigating the disappearance of Madeleine McCann are to begin searching on the ground in Portugal in the next few weeks.
Officers from the British investigative team will be in the resort of Praia da Luz with their Portuguese counterparts for the start of what the Metropolitan police said was a "substantial phase of activity on the ground".
But Mark Rowley, an assistant commissioner at the Met, warned that the operational activity did not amount to a significant breakthrough – but rather the "routine slog" of an ongoing investigation.
The activities were, however, informed by the investigation so far, and the sifting and analysing of what has been uncovered by the British team.
"Any actions we take are based on prioritising what is worth doing, whether it's about speaking to people of interest, looking at phone data or financial records, they are all based on prioritising," he said.
"There are many potential lines of inquiry. If we didn't think there were any fruitful lines of inquiry we wouldn't be where we are today. The only way to get on is to work through these systematically."
But for the first time Rowley admitted that the police investigation – which is being run with the Portuguese – may not result in any positive outcome. "We may go through every line of inquiry and all of them draw a blank," he said.
The Met was called in to carry out a review of the Portuguese inquiry by the prime minister, David Cameron, after an appeal by Madeleine's parents.
The investigative part of the Met inquiry began two years ago and has involved sifting through tens of thousands of documents, collecting phone data, identifying people of interest and identifying known sex offenders in the area at the time.
The British police have appealed again for media restraint as search teams begin work on at least three areas in and around the resort of Praia da Luz. These areas are understood to include wasteground behind a church, an area where roadworks were being carried out when the three-year-old British girl disappeared, and an area near the beach.
Madeleine went missing from her parents' apartment on the resort seven years ago on 3 May 2007 while her mother and father were having dinner with friends at a nearby tapas restaurant.
The British police inquiry has uncovered a string of sexual assaults and attempted sexual assaults in western Portugal. Between 2004 and 2006 there were nine sexual assaults and three near misses in the Algarve involving British girls aged six to 12. These include a sexual assault on a 10-year-old British girl in 2005 in Praia da Luz, two years before Madeleine was abducted.
Madeleine's parents are being kept up to date with developments in the investigation.
Searches in the areas will at first involve the use of ground penetrating radar to detect whether the ground has been disturbed. Excavations of the area could then follow. But Rowley said this investigation was moving at a much slower pace than any inquiry carried out on British soil because of the need for legal requests to the Portuguese and the need for the two jurisdictions to work together.
