The occupant of the flat at 17 Butterwick, believed to be a man called Giovanni, was not there when the fire broke out shortly after 7pm.

Norfolk Fire investigation officer Guy McCurley said: "The fire service was called at 7.30pm by passers-by, who had seen smoke coming from the flat, and arrived to find a bedroom well alight."

Firefighters from two Lynn appliances broke through the front door to make sure nobody was in the flat. They wore breathing apparatus and used hose reel jets to put out the flames within half-an-hour. Fans were used to dispel the smoke and fumes.

"The bedroom was extensively damaged by fire and there was severe smoke damage to the whole of the ground-floor flat," Mr McCurley said. "The occupier will have to be rehoused."

A 37-year-old man and a 15-year-old youth were arrested on suspicion of arson at the weekend. They have been released, without charge, on police bail until a later date.

Police are appealing for witnesses to the incident, or anyone with information about it, to contact Detective Constable Gail Morley at Lynn Police Station on 0845 456 4567, or Crimestoppers, anonymously, on 0800 555111.

Neighbours Vitor Silvano (26), his wife Andreia (22) and six-year-old daughter Ana were alerted to the fire at about 7.10pm by someone from another flat in the area.

Mr Silvano said: "I looked outside and there was a lot of black smoke coming from the door of number 17, and when we went round the other side of the flat I could see flames from the bedroom.

"There was a broken window where the bedroom is and we looked in but couldn't see anyone inside."

Mr Silvano said when the firefighters arrived, they advised his family to stay out of their home until it was safe. They were able to return indoors at about 8pm.

Mr Silvano saw the man living at number 17 outside his home later that evening. "He is a very nice person and when I came here about 17 months ago, he helped us," he said.

On Saturday morning, the fire-damaged remains of a bed lay in the courtyard outside the flats as both fire investigation and police scene-of-crime officers sifted through blackened debris in the affected premises and took photos.