Walliams is swimming The Big Splash Challenge to raise money for Sport Relief, which helps vulnerable people in the UK and in the world’s poorest countries, and aims to complete the swim in just eight days by swimming in two hour blocks.

The comic, who co-starred in the BBC show Little Britain, started to swim earlier today (5 September) at the river’s source near Lechlade, Gloucestershire, and is expected to finish the swim at Big Ben next Monday.

Walliams said: “Most people in the UK know the Thames, and most people think the idea of swimming in it is horrifying. And it is. It’ll be relentless. It’s a huge distance so I’ll need all the support I can get.”

If there is heavy rainfall during the swim Walliams will have to battle through around 39M cubic metres of raw sewage, which is released into the Thames every year. The water also contains of bacteria such as e-coli, salmonella and hepatitis.

Meanwhile, Walliams will also face strong tidal currents and oncoming river traffic.

Walliams warned: “It’s really not a safe environment to swim in. People really mustn’t try this, I have an incredible team of experts to help keep me safe.

“If there’s a lot of rainfall there’ll be a lot of sewage present so I’ve had to have loads of jabs. And aside from anything else, you can’t see anything. There’s zero visibility in the water – I won’t even be able to see the end of my arm.”

In 2006 Walliams swam the English Channel, a feat which raised £1M for Sports Relief. However, swimming the Thames is the equivalent distance of swimming the English Channel seven times – an additional 120 miles.

Walliams can be sponsored here.

Carys Matthews

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