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 today in relation to the three-year-old, who vanished in Praia da Luz in 2007.
It is thought that 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 Gerry and Kate, from Rothley in Leicestershire, dined with friends at a nearby restaurant.
South Wales Police confirmed that two of its search dogs and their handlers have returned to Portugal to help with searches.
Last month, two of the force's victim detection dogs, English Springer Spaniels Tito and Muzzy, travelled to the Algarve to help with the searches.
A force spokeswoman said two different dogs and their handlers have returned to Portugal to help the Metropolitan Police with their work.
After the last searches, police said it was the ''first phase of this major investigation which has been agreed with the Portuguese'' but that there was "still a substantial amount of work yet to be completed in the coming weeks and months''.
The McCanns' spokesman Clarence Mitchell said: "Kate and Gerry will simply not comment on any operational aspect of the Metropolitan Police work.
"What the police do and when they do it is a matter for them."
Mr and Mrs McCann are expected to return to Lisbon next week to attend their ongoing court case against former police chief Goncalo Amaral, whom they are suing over claims made in his book The Truth Of The Lie.
Last month the couple accused the former detective of a "blatant and cynical" attempt to hold up the trial after he removed powers from his lawyer on the day they were due to deliver personal statements about the effect allegations in his book had had on them.