Pugh was hospitalized after suffering vomiting spells, and was diagnosed with an ear infection and sinus problems that his support team attribute to river water.

"Lewis was pale, sweating and thoroughly despondent and took to his bunk for several hours. However by the afternoon he was feeling a bit better and decided to try a swim," wrote his backup team leader Major General Tim Toyne Sewell on the expedition's website.

Pugh (36) is tackling the daunting task in an effort to raise awareness of climate change. He has previously completed long-distance swims in the Atlantic, the Arctic, the icy Southern Ocean and the Indian and Pacific oceans.

By Sunday Pugh's website reported that he was almost back to full strength, and was determined to make up for lost time. "I am up and running again and am firmly determined to reach some hard targets today," Pugh wrote in his daily diary entry.

"Now that the river is open and wide there should be less risk of being hit by a boat, which was very much the case on the higher reaches."

Pugh started his swim on July 17, and aims to reach the mouth at Southend by July 29. Read Pugh's daily journal at www.investecthameschallenge.com