Portuguese authorities haven’t named the person of interest - in a case known as Christian B - but are acting on request of German authorities.
He has not been charged and denies any involvement in Madeleine’s disappearance.
Despite numerous appeals, millions of pounds in public donations and the backing of high-profile celebrities, Madeleine has not been found.
Here is a timeline of the main developments in the years since Madeleine vanished.
– May 3: Kate and Gerry McCann, from Rothley, Leicestershire, leave their children asleep in their holiday apartment in Praia da Luz in southern Portugal while they dine with friends at a nearby tapas restaurant.
Nothing is amiss when Mr McCann checks on the youngsters just after 9pm, but when his wife goes back at about 10pm she finds Madeleine missing.
Jane Tanner, one of the friends dining with the McCanns, reports having seen a man carrying a child earlier that night.
– May 14: Detectives take Anglo-Portuguese property developer Robert Murat in for questioning and make him an “arguido”, or formal suspect – but this is later withdrawn.
– August 11: Exactly 100 days after Madeleine disappeared, investigating officers publicly acknowledge for the first time that she could be dead.
– September 7: During questioning of Mr and Mrs McCann, detectives make them both “arguidos” in their daughter’s disappearance – but this is also later withdrawn.
– September 9: The McCanns fly back to England with their two-year-old twins Sean and Amelie.
– July 21: The Portuguese authorities shelve their investigation and lift the “arguido” status of the McCanns and Robert Murat.
– May 12: Mrs McCann publishes a book about her daughter’s disappearance, on Madeleine’s eighth birthday.
Scotland Yard launches a review of the case after a request from home secretary Theresa May, supported by prime minister David Cameron.
– April 25: Scotland Yard detectives say they believe Madeleine could still be alive, release an age-progression picture of how she might look as a nine-year-old, and call on the Portuguese authorities to reopen the case, but Portuguese police say they have found no new material.
– July 4: Scotland Yard confirms it has launched its own investigation, Operation Grange, into Madeleine’s disappearance two years into a review of the case. It has “genuinely new” lines of inquiry and has identified 38 people of interest, including 12 Britons.
– October 24: Portuguese police confirm that a review of their original investigation has uncovered new lines of inquiry, and they reopen the case.
– January 29: British detectives fly to Portugal amid claims they are planning to make arrests.
– June 3: Sniffer dogs and specialist teams are used to search an area of scrubland close to where Madeleine went missing.
– December 12: Detectives begin questioning 11 people who it is thought may have information on the case.
– September 16: The government discloses that the investigation into the disappearance of Madeleine has cost more than £10 million.
– October 28: Scotland Yard cuts the number of officers working on the inquiry from 29 to four.
– April 30: The McCanns prepare to mark 10 years since their daughter’s disappearance with a BBC interview in which they vow to do “whatever it takes for as long as it takes” to find her.
– May 3: Local media reports say Portuguese detectives are investigating a foreign paedophile as a suspect in the abduction of Madeleine.
– June 3: Police reveal that a German prisoner has been identified as a suspect in Madeleine’s disappearance.
– June 4: Scotland Yard’s Operation Grange, which had received £12.3 million in funding up to April 2020, is still a missing person inquiry as detectives have no “definitive evidence whether Madeleine is alive or dead”.
– May 4: Kate and Gerry McCann post a statement on the Official Find Madeleine Campaign website saying they still cling to the hope of seeing their daughter again as they prepare to mark her 18th birthday on May 12.
– April 21: Portuguese authorities declare a formal suspect.