Police have been investigating the prime suspect in the Madeleine McCann case for four years, Sky News has learned.

German prosecutor Hans Christian Wolters says police have been looking for information on the main suspect, known as Christian B, for a long time and “had already built up a fact file on him before we asked the public to help us”.

A year ago the prosecutor and Scotland Yard announced they had a new suspect and made appeals to anyone who knew him, his movements and lifestyle.

They didn’t name the suspect, but it quickly emerged he was a 43-year-old German drifter known, under German privacy laws, as Christian B.

Mr Wolters said: “By that time Christian B had been under investigation already for three years. We did not take the decision to go public lightly.

“Since then we have found a lot more pieces of the puzzle, but I cannot reveal what they are. We are making progress and have the same number of investigators working on the case, but we don’t yet have enough to charge the suspect.”

He added: “There is no time limit to our investigation, but we won’t keep it going forever.”

The prosecutor has said in the past he was conducting a murder investigation and that he has evidence that Madeleine died in Portugal, though he hasn’t explained to her parents Kate and Gerry McCann why he is so sure.

Christian B was living in Portugal in 2007 when three-year-old Madeleine vanished from her bed in her family’s holiday apartment in Praia da Luz on the Algarve coast.

The case against him is circumstantial – he is a convicted paedophile, he lived in the area, on the night she disappeared his mobile phone was in use nearby and the next day he changed the registration of one of his vehicles.

He’s also alleged to have confessed that he abducted Madeleine to a criminal associate during a bar conversation.

The associate, who was later arrested in Greece, contacted Scotland Yard and told them of Christian B’s alleged confession to him.

Scotland Yard detectives flew to Greece to interview the associate and they later changed the focus of the Madeline investigation.

Christian B is currently serving a seven-year jail sentence in Germany for the rape of an elderly American widow in Portugal in 2005. He denies any involvement in Madeleine’s disappearance.

His lawyer Friedrich Fulscher said: “My client is innocent in the Madeleine case. The prosecutor keeps saying he is guilty but he hasn’t interviewed him and hasn’t shown us any of the evidence he says he has.”

Mr Wolters is also investigating Christian B over three other allegations in Portugal: two sex assaults and the rape of an Irish holiday resort worker.

Mr Wolters said: “We are speaking with witnesses and interviewing a lot of people. We are making progress.”

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