The unusual arrangement - which will see a small number of Metropolitan Police officers work alongside their Portuguese counterparts - is seen as the last realistic prospect of uncovering what happened to Madeleine.
Arrests are not imminent in the McCann case but the multimillion-dollar review by 37 officers is believed to have uncovered new theories about events on the night she disappeared. Scotland Yard declined to go into detail but said they believed the youngster may still be alive because of a lack of evidence to suggest otherwise. She would be 10 years old now.
Madeleine was asleep in the family's holiday apartment as her parents dined at a nearby restaurant with friends when she was taken. Her parents, Kate and Gerry McCann, and their friends have been ruled out as suspects, leading police to believe she was snatched by a stranger.
After Portuguese police failed to find the girl, the McCanns employed private investigators. The Government is funding the review and will continue to pay for the full inquiry. Police have gone through nearly two-thirds of the papers in a laborious process of translation and logging more than 30,000 documents.
Some of the 38 identified have not previously featured in inquiries into the girl's disappearance, marked over the years by sightings across Europe and further afield, even in New Zealand, and by dogged campaigning by the girl's parents.
The Portuguese authorities have said they will only open the inquiry in the event of a substantial breakthrough. In a delicate diplomatic balancing act, Portuguese police will lead the latest inquiry but will be following up some of the 3800 leads identified by the Met in the review.
The officer heading Operation Grange, Detective Chief Inspector Andy Redwood, said: "We continue to believe that there is a possibility that Madeleine is alive. It's a positive step in our hunt for Madeleine that our understanding of the evidence has enabled us to shift from review to investigation."
- Independent