A 15-year-old boy has been arrested in Spain on suspicion of shooting dead his parents and ten-year-old brother following a row over bad school grades, police said.

The incident occurred on Tuesday evening in a rural area just outside Elche, about 20km from the southeastern port city of Alicante.

It was not discovered until last night by a relative who turned up to check on them, a police spokesman said.

"The mother's sister came to the house because she had heard nothing from the family and that's when her nephew told her he'd killed his father, mother and brother," he said, saying she had called the police who confirmed finding three bodies inside the house.

Police then arrested the teen, "a 15-year-old minor" who had been alone in the house with the bodies for three days.

He told them he had "argued with his mother over his school grades" then used his father's hunting rifle to kill her, then his ten-year-old brother and later his father.

The family was local to the area but not known to police, the spokesman said.