A Cardiff man has taken the plunge and set off on a mammoth challenge to swim the entire 220 miles of the River Severn in just 20 days.

Ross O'Sullivan, 25, began his swim this morning from the source of Britain's longest river in Llanidloes on the Welsh border.

The endurance swimmer has been training five times a week and covering distances of up to 17 miles to prepare himself for the feat.

His journey will take him through the towns of Shrewsbury, Worcester and Gloucester until the river meets with the sea in the Bristol Channel.

He has enlisted the help of friends who will keep an eye on him from a paddle boat and a car following the route on land.

But he will be making the journey without a tent, hoping businesses along the route will give him food and somewhere to sleep every night.

Credit: Ross O' Sullivan

In order to hit his target and complete the swim in 20 days he's planning to spend five hours in the icy water every day but admits it will be a struggle.

How do you really prepare yourself for 20 days of living in the wild, swimming 20km a day, down a river that is 354km long? It's probably impossible. It will be quite tough to hit this deadline, it's likely to run over to 25 days. >

Ross O' Sullivan
This is the route Ross will be taking over the next 20 days. Credit: Ross O' Sullivan

He says he's fully aware of the dangers involved which could include strong currents, pollution and floating debris.

But it's not going to stop this determined swimmer as he attempts to fund raising for the Motor Neurone Disease Association along the way.

Weather is pivotal. I've never prayed for rain before but it will make my life so much easier. Heavy rain means the river will flow a lot faster, allowing me to cover more mileage!

Ross O' Sullivan
Credit: Ross O' Sullivan

To follow Ross on his journey you can read his blog here or follow his progress on Twitter.