The German man identified as a suspect in the disappearance of Madeleine McCann is a convicted sex offender.
The man, who has not been named, is currently serving a sentence in a German prison for a sex crime.
He also has two previous convictions for "sexual contact with girls", Christian Hoppe, from Germany's Federal Criminal Police Office (BKA) told the country's ZDF television channel.
Mr Hoppe said police have not ruled out a sexual motive in Madeleine's disappearance.
Police broke the news on Wednesday evening that a suspect, known to have been in and around Praia da Luz on the Algarve at the time Madeleine disappeared on May 3, 2007, had been identified.
He is described as white, with short blond hair and is around 6ft tall and of slim build.
Madeleine vanished while she was on holiday in Praia da Luz with her parents, Kate and Gerry McCann, and her siblings Sean and Amelie.
She disappeared shortly before her fourth birthday and would now be 17.
In an appeal over the crime which they are treating as murder, Germany’s Federal Criminal Police Office (BKA) said: “The suspect lived more or less permanently in the Algarve between 1995 and 2007. For several years, he lived in a house between Lagos and Praia da Luz, among other places. He had several occasional jobs, among other things in the gastronomy business, in the Lagos area in this time period.
“In addition, there is information suggesting that he also earned his living by committing criminal offences, such as burglaries of hotel complexes and holiday flats as well as trafficking in narcotic drugs.
“Furthermore, the suspect was sentenced on numerous occasions to prison terms for sexual abuse of children in the past. This fact is probably not known to most of the contact persons.”
Police say a 30-minute phone call was made to the suspect's Portugese mobile phone around an hour before Madeleine is believed to have gone missing.
They have released two numbers - (+351) 912 730 680 is believed to have been used by the suspect and received a call from another Portuguese mobile - (+351) 916 510 683 - while in the Praia da Luz area, starting at 7.32pm and ending at 8.02pm on the night of May 3, 2007.
The suspect has also been linked to an early 1980s Volkswagen T3 Westfalia camper van with a white upper body and yellow skirting, registered in Portugal, and a 1993 Jaguar XJR6 with a German number plate which was seen in the Praia da Luz and surrounding areas in 2006 and 2007.
The day after Madeleine went missing, the suspect got the car re-registered in Germany under someone else's name, although it is believed the vehicle was still in Portugal.
Both vehicles have been seized by German police.
Police are appealing for anyone who may have seen the camper van in or around Praia da Luz on the night Madeleine went missing, the days before or weeks after.
Detectives also want to speak with anyone who saw the van together with the Jaguar, or individually, during the spring and summer of 2007.
Deputy Assistant Commissioner Stuart Cundy, from the Metropolitan Police's Operation Grange team, said: "This is a significant development, which is why we've made the decision to take this significant step of making a public appeal on some information that we would not normally talk about in our major investigations."
The Operation Grange incident room can be contacted via 0207 321 9251 or [email protected]
Scotland Yard is launching a joint appeal with the Federal Criminal Police Office of Germany (BKA) and the Portuguese Policia Judiciaria (PJ), including a £20,000 reward for information leading to the conviction of the person responsible of Madeleine's disappearance.