Police have identified a German paedophile as a suspect in the disappearance of Madeleine McCann 13 years ago.
The 43-year-old suspect, who has not been named by Scotland Yard, is currently serving a long prison sentence for unrelated offences in Germany.
The country’s Federal Criminal Police Office (BKA) is now leading the investigation, which it is treating as a murder inquiry.
German police said the suspect was a paedophile with multiple convictions, including for sexually abusing children.
He was living in Portugal at the time of Madeleine's disappearance and as well as working in catering, burgled holiday apartments and hotels and trafficked drugs.
The Metropolitan Police said they were still treating the case as a missing person investigation, and that they had no “definitive evidence” indicating whether Madeleine is alive or dead.
“As the years go on, we are realistic about what we might be dealing with but there is always hope,” said Deputy Assistant Commissioner Stuart Cundy.
“We are asking the public for help to prove or disprove whether this man was involved in Madeleine’s disappearance.”
“We retain an open mind about his involvement and we will follow the evidence wherever it may take us.”
Investigators are appealing for information on a campervan, car and mobile phone number linked to him that could be “critical” for the case.
The man was 30 at the time of Madeleine’s disappearance on 3 May 2007, when she had been on holiday with her parents in Praia da Luz, Portugal.
Then three years old, she went missing from the family’s ground floor apartment at the Ocean Club complex while her parents were at a nearby restaurant.
Investigators believe she left the apartment between 9.10pm and 10pm.
Little over an hour before, phone records placed the suspect in the area, where he received a half-hour long call from another Portuguese mobile number.
“The call received by the suspect was at 7.32pm and finished at 8.02pm, just over an hour before Madeleine was last seen,” said senior investigating officer Det Ch Insp Mark Cranwell.
“The caller we don’t believe was in the Praia da Luz area at that point.
“We are not saying that person is a suspect in this case. It’s someone we really need to identify as a key witness and we urge them to get in touch.”
Officers are also appealing for anyone who recognises either of the phone numbers, or had seen two vehicles linked to the man to contact investigators in the UK, Germany or Portugal.
One is a “distinctive” VW campervan and the other is a Jaguar, which was re-registered in another name in Germany on the day after Madeleine’s disappearance.
“We know he was in that area in the campervan in the weeks and days before, and afterwards,” Det Ch Insp Cranwell added.
“Some people will know the man. I’m appealing to you directly. You may be aware of some of the things he has done, he may have confided in you about the disappearance of Madeleine.
“More than 13 years have passed and your loyalties may have changed. This individual is in prison and we are conscious that some people may have been concerned about contacting police in the past. Now is the time to come forward.”
There is a reward of £20,000 for information leading to the conviction of the person responsible for Madeleine’s disappearance.
The suspect is described as white, about 6ft tall and of a slim build, with short blond hair.
He is currently 43 but in 2007 he was 30 and police said he may have looked between 25 and early 30s.
Scotland Yard said they started investigating the man as a suspect after a police appeal on the 10th anniversary of Madeleine’s disappearance in 2017 resulted in “significant information” being received.
But investigators admitted the name was previously known as one of 600 people who were considered as potentially significant.
“Before 2017 he was not a suspect and this a direct consequence of that new information [from the appeal],” Mr Cundy said.
“Our main line of inquiry is this suspect.”
The senior officer said the 12 May would have been Madeleine’s 17th birthday, adding: “Her disappearance is etched on our memories and none of us can imagine what it must be like for her family not knowing what happened.
“The Met Police has always said that we would do what we could to find out what happened and conduct our enquiries with a determination to uncover the truth.”
Officers said they were considering a “number of motives” but would not confirm if Madeleine was suspected of being abducted in the campervan.
In a statement her parents Kate and Gerry McCann said: “All we have ever wanted is to find her, uncover the truth and bring those responsible to justice.
”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.“
Police are seeking information on:
A VW T3 Westfalia campervan. Early 1980s model, with two tone markings, a white upper body and a yellow skirting. It had a Portuguese registration plate.
A 1993 British Jaguar, model XJR 6, with a German number plate and registered in Germany.
The phone number +351 912 730 680, which was being used by the suspect.
The second number is +351 916 510 683, which called the suspect on the night of Madeleine’s disappearance.
The Operation Grange incident room can be contacted via 0207 321 9251 or email@example.com.