Photo: NEWSTEAM
Lynn Jones, from Shrewsbury, Shropshire, said Otto, a dachshund cross terrier, was put down by a vet on Thursday.
Otto, who was officially crowned the oldest dog in the world by the Guinness Book of Records in October last year, had been ill for some weeks.
Mrs Jones, 54, said she had hoped it was just an ulcer, but a vet delivered the news yesterday that a large tumour had developed in Otto's stomach.
Mrs Jones and her husband Peter, 68, took their pet – who they have had since he was just six weeks old – to be put down this morning.
Mrs Jones said: ''I have cried all morning, I am devastated."
''We found out yesterday that he had a very large tumour in his stomach and last night I felt that he was trying to tell us something.
''He couldn't sit or lie down comfortably and he was trying to tell us that he'd had enough, so we made the decision to end his suffering and took him to the vet this morning.
''He died at 10.10am and I feel like I've lost my right arm."
Tributes to Otto, who would have been 21 on February 14, began to pour in online as the news of his death reached fans.
The dog had his own page on social networking website Facebook – attracting 669 members. More than 50 messages of condolence were added to the page today.
Otto will be cremated on Friday.
England with Shropshire highlighted in red.
The world's oldest dog, who answered to the name of Otto, has died at the age of 20. Otto, who was a Dachshund, was born and lived in Shrewsbury, in the county of Shropshire in England.

The dog had to be put down by a vet on Thursday after it was discovered that the animal had developed stomach cancer.  The dog's date of birth was February 14, 1989, which means that Otto would have been 21 years old 31 days after his death occurred.  Lynn Jones and her husband Peter had both owned the dog since he was six weeks old.  Otto was officially declared the oldest dog in the world by the Guinness Book of Records in October 2009.  He was cremated on Friday.

Now Lynn Jones has payed tribute to her pet. "I have cried all morning, I am devastated," she said. "He died at 10:10AM and I feel like I've lost my right arm. I know I sound like a wuss, but he was like the son I never had, my baby boy. I am glad he is not suffering any longer but I wanted him with me forever. We found out yesterday that he had a very large tumour in his stomach and last night I felt that he was trying to tell us something. He couldn't sit or lie down comfortably and he was trying to tell us that he'd had enough, so we made the decision to end his suffering and took him to the vet Thursday morning."