French authorities on Wednesday opened an investigation into the shocking case of a mother and her four children who appear to have been kept locked up for at least a year in squalid conditions in their council flat.

The father, the only member of the family seen by neighbours in over a year, has been detained in a psychiatric hospital since the family was discovered by emergency services who responded to a call from the mother during the night of Friday-Saturday.

The children, aged 14, 17, 19 and 20, and described as having learning difficulties, were found prostrate in the mould-covered apartment in a block of social housing in the rundown port of Saint-Nazaire on France's Atlantic coast.

Neighbours thought the mother, Christine Barreteau, 47, and the children had left her partner, Dominique Barreteau, 51. He was regularly seen putting out rubbish or coming back with shopping but never responded to any greetings.

The organisation that runs the housing block said they had no indication of the flat having got into such a dilapidated state, despite bolts having been attached to the outside of the door and windows.

But neighbour Pascale Le Gall said she had alerted officials to the foul smell coming from the flat and to an incident in which she had been threatened by the father.

A fifth child of the family, an elder daughter, was helping police with their enquiries.