German newspaper Braunschweiger Zeitung have reported the suspect, who has been partially identified as Christian B by local media due to the country's strict rules on the naming of criminals, was convicted of rape in Braunschweig District Court in December past year.
Madeleine, then aged three, disappeared from her hotel room in 2007 during a family holiday in Praia da Luz, a small town in the southern Algarve where some foreign visitors end up living all year round.
Police said the suspect, whom they have not named, is now serving a lengthy prison sentence for another crime.
Inga Gehricke vanished from a nearby forest in Saxony-Anhalt on May 2 2015.
Now, a German man has been identified as a key suspect in the case.
In a statement following the announcement of the new suspect, but before the prosecutor said she was assumed dead, Madeleine's parents, Kate and Gerry, said in a statement: "We welcome the appeal today regarding the disappearance of our daughter, Madeleine".
Scotland Yard said his name was already known to its investigation, but he did not become a suspect until after information was received as a "direct consequence" of a 2017 appeal.
German sex offender identified as key suspect in Madeleine McCann disappearance
German authorities are treating the toddler's disappearance as a murder investigation but Britain's Metropolitan Police has always considered the case a missing persons inquiry. "We have been working with colleagues in Germany and Portugal and this man is a suspect in Madeleine's disappearance".
As the case was investigated, Madeleine's parents were suspected to be related to the girl's vanishing, but were justified in June 2008, when Portuguese police halted the case.
The now-retired police chief said British detectives had been focusing their investigation on two German paedophiles.
They said: "We will never give up hope of finding Madeleine alive but whatever the outcome may be, we need to know, as we need to find peace".
German police have released images of the farmhouse in an appeal for witnesses.
He added: "They are trying to maintain as normal a life as possible under the circumstances, continuing their medical work where necessary and bringing the twins up as best they can, while shielding them from all the attention".
German prosecutors said the suspect was living in the Algarve between 1995 and 2007, where he funded his lifestyle by committing crimes.
Anyone who has seen the VW T3 camper van, or the Jaguar XJR 6 vehicles in the area of Praia de Luz should contact the police immediately.
Stocks register gains on cautious hopes of economic recovery
That raises optimism that Friday's more comprehensive jobs report from the USA government may also not be as bad as feared. Treasury prices remained under pressure, as the benchmark 10-year Treasury yield advanced to 0.826% from 0.761% Wednesday.