A six-year-old boy was in critical condition in hospital after his parents "punished" him by leaving him outside on the freezing balcony of their flat in Brussels wearing only his pyjamas for 15 hours.

The French mother, 31, and Belgian stepfather, 21, face charges of torturing their son after making him stay outside in temperatures that did not rise above 3C as punishment for helping himself to some food in the family fridge.

His twin sister was also taken to hospital for treatment. Both children showed signs of abuse and malnutrition, the Brussels prosecutor's office said.

The boy was unconscious and suffering from hypothermia when ambulance workers were called to the flat in the working class Saint-Josse area of the Belgian capital on Monday evening, it said.

Local media said he had been forced out onto the balcony at around 5 am the day after Christmas Day and stayed there until around 8 pm.

"The step-father has confessed without realising the seriousness of the facts, while the mother is still in denial," the prosecutor's office said in a statement.

Both were being held in police custody.

The balcony was at the back of the apartment building and was not visible from the street, which local media said explained why no-one had called police or emergency services sooner.