Comedian David Walliams is falling behind schedule in his charity eight-day swim along the Thames.
The Little Britain star suffered severe vomiting and diarrhoea during the third day of the 140 mile (225km) swim.
He had to end the day at Wallingford Bridge, Oxfordshire, about five miles short of his target of Goring.
Walliams said: "I always knew there was a risk that taking in the water could cause problems, but now it's happened it's still hit me really hard."
He also had a high temperature and struggled to keep up his pace on his Big Splash Challenge for Sport Relief.
Before he set off from Abingdon Bridge on Wednesday morning, the Sport Relief Twitter feed said: "David has woken up with an upset stomach - Thames Tummy."
He was assessed by the medical team at each stop during the day as the condition also made him reluctant to eat and sapped him of energy.
He has already raised more than £240,000, something which the comedian said had given him a "real boost" despite his illness.
Crowds gathered at Wallingford Bridge to cheer him as he finished the final stretch of Wednesday's swim at about 18:40 BST.
Walliams also ended the first day of his swim early, 2.3 miles (3.7km) short of his planned stop at Northmoor Lock, and in "a lot of pain".
On Tuesday he began at dawn to make up for the lost distance and swam through the dark to complete the 21 miles to Abingdon, where he arrived at 20:20 BST.
He started at the river's source near Lechlade, Gloucestershire, on Monday morning, where hundreds of people turned out to cheer the star on, and will finish at Big Ben.
Day two saw Walliams swim through Oxfordshire, via Eynsham, Oxford and Osney, before finishing at Abingdon.
Hundreds of people have been gathering along the route to offer support.
To avoid disappointing fans who had gathered to see the star at Northmoor Lock, west of Oxford, on Monday evening, he travelled by boat to greet them.
During the swim, Walliams will battle powerful and unpredictable currents and undertows and will burn the equivalent of 4,400 calories every day.
As well as navigating the busy river traffic, the comedian will have to deal with murky water that harbours a cocktail of bacteria such as E.coli, salmonella and hepatitis.
Walliams will also have to avoid the 39 million cubic metres of raw sewage that finds its way into the Thames every year after heavy rainfall.
The distance is equivalent to swimming the Channel seven times. Walliams swam the Channel successfully for Sport Relief in 2006.
He has also swum the Straits of Gibraltar and last year cycled from John O'Groats to Lands End, also for Sport Relief.