Thomas, the oldest tortoise in the UK, had to be put down after being attacked in the garden of its owner's home in Guernsey by the rodent.
Living through the reigns of six British monarchs, at first it looked as though the lettuce-loving tortoise would recover from its injury.
The tortoise spent five days on a course of strong antibiotics, being fed through a tube, the 'Daily Express' reported.
"We are very upset, distraught actually," she said.
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"Thomas was like a member of the family. He was bitten on the leg by a rogue rat in the garden and the wound became so inflamed that it started pushing into his shell.
"He wasn't eating or appearing to get any better, so a week later, as a family, we made the decision to put him to sleep," said Le Gallez.
Fifty-four-year old Gallez inherited Thomas 35 years ago from her late cousin Grace Hilditch. Grace had been given the tortoise in 1922 as a present from her father, who was friends with a London Zoo reptile keeper.
The tortoise hid a dark secret under its shell for most of its incredibly long life - it was actually a female.
It was only when Le Gallez became its owner in 1978 that a vet discovered the staggering truth, nearly a century after Thomas was born, the report said.
By then the tortoise was already a national institution in UK, so the name stuck.