A British family’s “cuddly” pet dog mauled their newborn baby to death — possibly because he was “jealous” of the days-old new arrival, according to a relative.

Chow Chow crossbreed Teddy — so named because he looked as cuddly as a teddy bear — leaped out of his garden pen Sunday and raced in to attack 12-day-old Elon as he slept in a basket at home in Doncaster, his uncle told DailyMail.com.

Elon’s dad, Stephen Joynes, 35, and one of his young stepsons raced in to try to save the baby, who weighed less than seven pounds, but Elon was pronounced dead later in a local hospital.

“It’s your worst nightmare,” the uncle told the outlet, saying his sister — Elon’s 27-year-old mom Abigail Ellis — was distraught.

“Teddy’s not a bad or dangerous dog,” he insisted. “I think he was jealous when the new baby came along and all the attention he was getting. Or maybe he thought the baby was a toy, he was a tiny thing,” he told the Mail of the tragedy.

Elon’s parents were both arrested on suspicion of manslaughter by gross negligence, and released on bail while the case is investigated.

Police put down the dog.

The uncle, who declined to be named, insisted the attack was “simply a tragic accident,” adding, “It will affect them all forever. They’ll never get over it.”

“What happened is heartbreaking. I never even got to see my new nephew,” he told the Mail while laying flowers at the house.

Friends told The Mirror that mom Ellis was already heartbroken after her previous fiancee — the father of her three other children — died two years ago from heart problems.

“Baby Elon was her ray of hope amid the tragedy,” a friend told the outlet, saying he was supposed to be her “fresh start.”

“This will break her,” the friend said.