It's the first formal development in the case in Portugal in several years.
A fire brigade spokesperson said police were preparing to search at a dam in the southern region of Algarve about 50 km (31.07 miles) inland from a beach resort where Madeleine went missing from a rented apartment while on a family holiday.
Reuters video showed Portuguese police had set up an operation centre around the reservoir.
A source familiar with the investigation told Reuters the operation in Silves municipality was being carried out at the request of German authorities, who last year formally identified German man Christian Brueckner as an official suspect in the disappearance of McCann.
German officials had no immediate comment.
Portugal's investigative Judicial Police in charge of the operation declined to comment.
Brueckner, a convicted child abuser and drug dealer, is behind bars in Germany for raping a 72-year-old woman in the same area of the Algarve from where Madeleine went missing.
German police said in June 2020 that Madeleine was assumed dead and that Brueckner was likely responsible for it.
Brueckner has denied any involvement and has not been charged with any crime related to it.
A German court last month threw out additional charges against him related to other Portuguese child rape and sexual assault cases that meant the German prosecutors investigating the disappearance of "Maddie" no longer had jurisdiction to investigate it.
Reuters