Kim Broughton, 44, had collected the egg from one of the hens she keeps but didn't noticed its rare shape until she started preparing the first meal of the day.
Kim, from Latchingdon, Essex, told her local paper: "I did not notice that the egg was round until I got it inside the house and washed it.
"It's absolutely amazing I have never seen anything quite like it. It really is perfectly round. The eggs tend to get bigger as the hens get older but I've not had anything like this before."
Britons eat more than 11 billion eggs a year but only a handful of perfectly round specimens have been found.
Kim, who keeps 15 hens at her family home hopes to raise at least that much auctioning her's on eBay to raise funds for the Cystic Fibrosis Trust.
She has named the hen that laid the egg Ping Pong - as in table tennis ball!