The firm mass the size of a large tennis ball was actually two mummified foetuses which had been floating in Missy's abdomen for more than three years.

"I had never seen anything like it before, nor had my boss Rick Prowse who has been a vet for more than 30 years," Austinmer's Dr Jarrod Coleman said.

Missy's owner Dawn Nobes from Helensburgh said she and husband Bill adopted Missy three years ago from the pound.

"We have a male dog in the house but he has been desexed so the pregnancy must be older than three years," she said.

"The only reason we took Missy to the vet's was because she had become a bit down and listless."

Dr Coleman said he and colleagues tossed around a number of theories.

"I think the strongest one is that the embryos escaped somehow through the tiny gap between the fallopian tubes and the ovary.

"In humans the embryo can get wedged in the fallopian tube which is an ectopic pregnancy, but this is different again because it escaped into the abdomen where it had no chance of ever being born."

He said the foetuses developed to seven weeks and five weeks - only weeks short of normal gestation period.

Veterinarian Rob Zammit said the phenomena was extremely rare but in his experience it was caused by a ruptured uterus.

"The foetus cannot survive in the abdomen so the body mummifies it in a casing of fibrous tissue which allows the body to continue to function normally," he said.

Dr Zammit said ruptures were normally caused by trauma such as a car accident.