Three prisoners used a table leg to pierce a hole in the roof of their cell in a 14th century French penitentiary and escape in the early hours of New Year's Day, a regional prosecutor said on Wednesday.

The three climbed onto the roof of the Colmar prison in eastern France and from there entered an adjoining courthouse, where they fled through a side door.

The escape highlighted overcrowding and the advanced state of disrepair of an institution originally built in 1316 as a convent, according to prosecutor Bernard Lebeau.

"It appears that the ceilings in the cells are made of a crumbly material that was attacked with a makeshift tool made of objects from the cell itself, notably a table leg," he told journalists in Colmar.

The prisoners, aged 19 to 24, had been jailed on charges of assault and vandalism. Two were detained awaiting trial while the third was serving a two-year term.

Poor conditions at the Colmar prison were flagged last week in an independent report commissioned by a lawyer, who said he would sue to make local authorities repair the facility.