Police in Greece have launched an international appeal to establish the identity of a blue-eyed blonde girl found living with a Roma couple to whom she is not related.

They believe that the four-year-old, named Maria by the Roma, may have been abducted or trafficked into the country from northern or eastern Europe.

She is currently living with a charity.

The parents of Madeleine McCann, the British child who went missing on a family holiday in Portugal in 2007, said the case had given them "great hope" that their daughter may one day be found alive.

After raiding the camp near Farsala in central Greece, investigators became suspicious when they noticed a blonde child who looked nothing like the couple who claimed to be her parents. Nor did she bear any resemblance to other children in the camp.

Costas Giannopoulos, director of the charity Smile of the Child, which is looking after the girl, praised the alertness of one investigator in an interview on Greek Skai TV. "She saw a little blond head poking out from under the bedclothes. It struck her as odd, and that's how it all started," he said.

When they asked to see the couple's papers, the information was found to be inconsistent, with the couple having registered different numbers of children with different regional authorities.

The woman also claimed to have given birth to several children in a 10-month period, and when questioned by officers, the couple gave conflicting accounts of how they came to be in custody of Maria.

The 39-year-old man and 40-year-old woman were arrested on suspicion of child abduction.

A DNA test has proved that the child is unrelated to them.

The couple's lawyer, Marietta Palavra, has subsequently said that they took the child in out of charity, after an intermediary gave the her to them when the mother was unable to look after her.

"Just because (the suspect) had forged documents, it doesn't make her a kidnapper," she said. "The couple loved the girl as if she were their own."

Drugs and firearms were seized in the raid.

The child only speaks Roma, but appears to understand Greek, and is being communicated with using sign language.

Greek police have asked for the assistance of Interpol and children's charities to help identify the child.

Panayiotis Pardalis, a spokesman for Smile of the Child, said: "It was obvious that she was not a Roma girl.

"She was afraid and under some psychological pressure when she arrived. Colleagues have been trying to communicate but are struggling.

"She seems to understand Greek but cannot speak it. She was living under bad conditions and was very dirty but is now safe."

Giannopoulos said the child was confused by her change of environment, and was undergoing medical examinations, but seemed to be in good health.

Owing to her blonde appearance she had been used to beg in the nearby town of Larrissa, he said.

He said the case raised serious questions about Greece's child registration system.

"We are shocked by how easy it is for people to register children as their own," he said.

"There is much more to investigate, there are other registered children that were not found in the settlement, and I believe police will unravel a thread that doesn't just have to do with the girl."

In Greece, adults need only make a formal declaration backed by two witnesses to be officially acknowledged as a baby's parents.

Anyone with information is asked to call the European Hotline for Missing Children (116 000), Smile of the Child (+30 210 76 09 550) or send an email to '116000@hamogelo.gr'.