Sunday, 1 July 2012:

An attempt by British-Australian woman Penny Palfrey to swim unassisted from Cuba to Florida ended in failure early on Sunday after she had to be pulled out of the water unable to cope with a strong ocean current.

"Penny Palfrey had to be pulled out of the water, after swimming for more then 40 hours," the team said in a tweet.

It said Palfrey had to abort her bid to reach the Florida Keys at about midnight (1400 Sunday AEST) due to a strong southeast current "that made it impossible for her to continue her swim."

"Penny is presently on her escort boat being taken care of by her crew," the team said.

The mother of three and grandmother of two had been seeking to become the first woman to complete the historic feat without a protective cage.

The 49-year-old, who left the Cuban capital of Havana shortly after sunrise Friday, had already covered 139kms, or three-quarters of her itinerary when she ran into trouble.

She was just 43kms southwest of Key West, her final destination.

By making it this far, she has already broken her own world record of the longest unassisted ocean swim of 59.64 nautical miles (110.45 kilometres), her team members said.

Despite her failure, her Facebook page quickly filled with comments by well-wishers, expressing support and saying that making it so far was already a great achievement.

The 166km trek from Havana to Key West is considered risky and fraught with unexpected dangers even by the best of athletes.

Earlier, Palfrey's support team reported that she had suffered "constant" jellyfish stings overnight and that her mouth was "very sore and painful" while a school of hammerhead sharks was briefly sighted below her.

Palfrey was seeking to accomplish the feat in a "call for friendly relations between the peoples of the United States and Cuba," according to the Cuban foreign ministry.

But even Cuba's national commissioner for swimming, Rodolfo Falcon, sounded a note of caution when he told AFP that "sea conditions are not similar to the pool, where she trained for many hours."

"At sea, the salt water weighs you down," said Falcon, who won a silver medal at the 1996 Olympic Games.

Susie Maroney, a former Australian marathon swimmer, swam from Cuba to Florida in 1997 when she was just 22, but she used a shark cage.

Veteran US endurance swimmer Diana Nyad, 62, has tried, and failed, to complete the trek three times, twice without a shark cage.

Her latest attempt was in September, when she quit two-thirds of the way into the crossing after suffering dangerous jellyfish stings. She plans to try again later this summer.

Two yachts, a kayak and a boat were part of Palfrey's support team. The vessels carried ultrasound equipment to ward off sharks.

Palfrey, who was born in Britain and moved to Australia at the age of 19, is among the most accomplished open-water swimmers in the world and has completed swims in the Caribbean and Pacific without a shark cage.

Two years ago, she crossed the Strait of Gibraltar between Spain and Morocco in three hours and three minutes, setting a new record for women.

Last year, Palfrey - who began swimming at age nine - swam from Little Cayman to Grand Cayman Island, again without a cage.

AAP