Kate and Gerry McCann could face criminal charges for using a private security company to investigate the disappearance of their daughter, a senior Portuguese judge has said.

The couple have been secretly receiving advice for more than four months from Control Risks Group, which employs former members of the British intelligence services and special forces.

News of the investigators’ role has led to deep concern amongst the authorities in Portugal, where it is illegal for private detectives to become involved in criminal cases.

A source close to the McCann’s legal team has confirmed the involvement of Control Risks Group but insisted it is simply providing advice on the hunt for Madeleine rather than becoming actively involved in searches in Portugal.

He said: “You can assume that they are doing some things that the Portuguese police can’t do. Nothing illegal is being done in Portugal.”

Antonio Martins, president of the Association of Portuguese Judges, told the 24 Horas newspaper: “It is still up to the state to carry out criminal investigation. That kind of activity has no legal standing. Anything that results from private investigation has no substance.”

The judge said that Mr and Mrs McCann, both 39, from Rothley, Leicestershire, could be charged with “obstruction of justice” if prosecutors find evidence of a parallel investigation. The couple are already official suspects in the disappearance of their daughter from a holiday apartment in Praia da Luz 145 days ago.

A high-ranking officer in the Polícia Judiciária, which is investigating the disappearance of Madeleine shortly before her fourth birthday, said that the investigators could be arrested if they are found operating in Portugal. “If they come here they will be running a serious risk of being arrested,” he said.

Carlos Anjos, President of the Judicial Police Inspector's Union, accused to Mr and Mrs McCann of using “diversion tactics”. Mr McCann had previously said that they had decided not to use private detectives.

“This is just another McCann strategy who today say one thing and tomorrow something completely contrary,” said Mr Anjos “This can only be another diversion tactic from the McCanns.”

Meanwhile, the only other official suspect in the case yesterday denied a report that he was seen outside the Ocean Club resort on the night Mrs McCann claimed that Madeleine had been abducted from her bed.

Robert Murat, 33, has insisted he was at home with his mother at their villa 100 yards from the resort where Madeleine was staying. Mr Murat later worked as an official translator for the police investigating Madeleine’s disappearance.

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