Palfrey, a 49-year-old grandmother, was plucked from the waters of the Florida Straits at about midnight after setting out from Havana on Friday and swimming for more than 40 hours, Andrea Woodburn, a member of Palfrey's logistical support crew said in an e-mail.
Sent to reporters the e-mail said Palfrey had to be pulled out of the sea "due to a strong southeast current that made it impossible for her to continue her swim."
Woodburn did not give the exact location where Palfrey gave up her bid, but she had already made it well past the halfway point across the dangerous body of water that separates communist Cuba from the United States.
Andy Newman, a tourism official in Key West, at the southernmost tip of Florida, said Palfrey had been taken to the Lower Keys Medical Center on nearby Stock Island hours after she was pulled from the water. But members of her support crew were not immediately available for comment.
Palfrey, who was born in Britain but lives in Australia, had initially hoped to complete the crossing and arrive somewhere in southern Florida within 40 to 50 hours.
Her swim followed two unsuccessful attempts last year by American marathoner Diana Nyad, now 62, to cross the Florida Straits, which are known for tricky currents and unpredictable weather.
The swim was completed successfully by Australian swimmer Susan Maroney in May 1997, but unlike Nyad and Palfrey she used a shark cage.