Officers from Scotland Yard's Operation Grange are thought to have joined their Portuguese counterparts as they interview people of interest this week.
Detectives in the Algarve were expected to begin interviewing a number of key suspects yesterday in relation to the three-year-old, who vanished in Praia da Luz in 2007.
It is thought UK officers are sitting in on the interviews, which are believed to be taking place in Faro, but will not ask questions themselves.
All those being questioned are reportedly Portuguese nationals, although some are Russian-born, and have been identified by British police as suspects.
It is thought they include three workers from the Ocean Club in Praia da Luz who have been linked to a string of burglaries in the area before Madeleine's disappearance.
Neither the Metropolitan Police nor officers in Portugal would make any official comment on the reports.
Last month British officers, accompanied by their Portuguese counterparts, carried out searches of three areas of land near to where Madeleine went missing.
She had been in a holiday apartment as her parents, Kate and Gerry, who is originally from Glasgow, dined with friends at a nearby restaurant.