Comedian David Walliams has begun the final leg of his bid to swim the 140 miles of the River Thames for charity.The Little Britain star, who entered the water at Teddington in south-west London, has 15.5 miles (25km) to swim to the finish at Westminster Bridge.Walliams is continuing his Big Splash Challenge for Sport Relief despite being warned not to swim in the river.Thames Water said 500,000 cubic metres of raw sewage had entered the London section of the Thames last week.Sport Relief said Walliams had received several inoculations to protect him from diseases carried in the river, and his health was constantly being monitored.He began at the river's source in Gloucestershire on 5 September but suffered a bout of "Thames Tummy" which left him with a high temperature.Walliams, 40, said he had recovered from the illness but was wary of the challenges ahead.'On Monday, he is tackling arguably the toughest stretch of the Thames where it becomes tidal, with strong currents and undertows, and has busy river traffic.He said the last leg of the swim "will see me entering the tidal stretch of the Thames with all the dangers that brings, so I'll need to be strong".Speaking from Richmond to Chris Moyles on BBC Radio 1 he said: "Every part of my body aches and I am going a bit loopy."I need a nice lie down and a massage by you."He added he would have to stop later as the tide was turning but still hoped to make it to Westminster Bridge for 18:00.The comic said on Sunday: "The two things that have really kept me going so far are the updates of the fundraising totals I'm given and the stunning support I've been given by the public."I never expected this to catch the public's imagination like it has - there seems to have been a real carnival atmosphere around it, which is lovely."Walliams, who also swam the English Channel in 2006, has so far raised £900,000 for Sport Relief.On Saturday, Thames Water said 500,000 cubic metres of raw sewage had entered the river due to heavy rain last week and tidal conditions mean it could take as long as a month for all of the waste to reach the sea.About 39 million tonnes of sewage is discharged into the river each year, due to a lack of capacity in London's Victorian sewerage network. From / BBC