David Walliams is being tested for a life-threatening waterborne disease after he developed symptoms on his River Thames swim.

Since he completed the epic 140-mile swim, the 40-year-old comedian has been suffering from symptoms that match up with those of Weil’s disease, such as fever and extreme muscle and joint pain.

Millions contract the illness every year, through water that has been contaminated by infected animal urine. In the most extreme cases, the bug can kill.

Double Olympic gold medal winner Andy Holmes is believed to have died from the illness last October, aged 51.

Walliams, who swam the Thames for charity Sport Relief, is being tested for the disease in London.

A source said: “The first signs were during the swim — the sudden fever, the diarrhoea and the rash.

"What was especially significant was the sudden stop of the illness only to return a few days later.

"The extreme and continued muscle and joint pain and the fact he is still extremely fatigued and feverish show all the classic signs of Weil's.

"He is undergoing tests but at least he's in good hands."

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Walliams is also said to have torn a disc in his back, making it difficult for him to walk. He has received treatment in hospital for this.

But he finished the eight-day swim and raised a not-too-shabby £1 million for the charity.

Walliams, who is married to model Lara Stone, has had to cancel several appearances including a guest slot on Jonathan Ross’s ITV chat show and filming for A League of Their Own with James Corden.

After the mammoth swim, the comedian said: “A bath is the only water I want to see for quite a while.”