French social services have found four children aged between two months and six years in a locked flat that they appeared not to have left since birth.

The eldest two children could barely talk or walk and had never been to school; none had ever seen a doctor. Investigators said the parents had "absolutely no awareness" of the situation.

The youngsters were sleeping on mattresses in one room in the flat on a housing estate in the rundown Paris suburb of La Corneuve, a source close to the investigation told BFMTV. "These are wild children, who have developed of their own accord," the source added. "It appears they never went out, or at least very rarely. Their neighbours didn't even know they existed."

There was no evidence that children were living there in any other part of the flat.

According to the French radio station RTL, which broke the story on Thursday, the children were discovered last month after the 27-year-old mother took the two-month-old baby girl to hospital, where staff concerned about the infant's health called in social services.

The mother and 33-year-old father have been taken into custody and face charges of child cruelty. The children have been placed in foster care. There was no evidence that they had been beaten or sexually abused.

"The flat wasn't very clean or tidy, but it wasn't a tip either," the source told RTL. "There was nothing to suggest there were young children living there."

The parents, who were unemployed, were collecting child benefit. If found guilty of cruelty they face up to seven years in prison and a €100,000 fine each. Neighbours in the 15-storey tower block told journalists they often saw the parents but were shocked to discover there had been children living in the apartment. "Sometimes I saw the father, but I've never seen any children," said one neighbour.