Tarnia Howe, of Newbury, Berkshire, spotted the spider amongst grapes she and her two sisters had been eating.

The schoolgirl called her father, who felt he recognised a red marking as similar to that on a black widow.

Sainsbury's chiefs have apologised for the shock discovery and have sent the spider to a laboratory for tests.

Suppliers checked

The fruit itself had been bought at the family's local branch of Sainsbury's on Friday.

The spider was found the next day, amongst the grapes in the refrigerator.

Mother Rosemary Howe said that a red marking on the abdomen had convinced her husband that the creature was a black widow, the bite of which can cause nausea, muscle cramps and fever in humans.

"My husband had been in the States and had seen them there. We also looked it up on the internet to check it," said the 40-year-old shop assistant.

Sainsbury's is trying to check how the creature got into the fruit, including tracking it back to the growers in Mexico.

A spokeswoman for Sainsbury's said: "Because it's a very rare occurrence that's not to say that when it happens it's pleasant and that's why we've apologised."