National Sea Rescue Institute (NSRI) spokesman Craig Lambinon said a number of people had seen a shark attack a 76-year-old woman shortly after 7 a.m.

"The shark came out of the water and attacked, came around and attacked again a second time and then a third time. It was thrashing her body in the water and then swam off with her.

"She had been swimming in the same place every day for 17 years," he said.

Lambinon said a search helicopter later spotted a 20-foot shark in the area.

Paul Dennett, the vice commodore at the nearby False Bay Yacht Club who witnessed the attack, said: "I saw this huge shark thrashing at something in the water, I thought it was a seal but then saw that it was a lady"

"It left her floating in the water, and there were massive amounts of blood. Then it ... came around and its whole mouth came out of the water and it took her down."

Dennett said the woman had been swimming about 15 yards off the shoreline at the time of the attack. The search for her body was called off at mid-morning. Surrounding beaches were closed.

"All efforts to find the lady have been exhausted by a wide and thorough search, we are hoping that police divers will be successful in their efforts (to find her body)," the NSRI said.

Lambinon said it was highly unusual for a shark to repeatedly attack a person, and he suspected that fish released by fishermen in the area may have created a feeding frenzy.

A great white killed a surfer in the area in September 2003, while a teen-ager lost his leg in an attack in 2004.