A St Albans swimmer is diving deep into UK rivers on a mammoth 50km challenge to raise cash for charity.

Between July 13 and September 28, Mark Smith will swim 50km over seven weeks down five rivers, including the River Adur in Sussex; the River Thames; River Spey in Cairngorms; River Wye in Herefordshire; and River Mawddach in Snowdonia.

Each swim will last between three to six hours over distances ranging from 6.5km to 14km - as 10km is classed as a swimming marathon, the 14km slog is an ultra marathon.

If successful, Mark believes he will be the only UK swimmer to complete the challenge this year.

He is hoping to raise £2,000 for Thrombosis UK after suffering from a blood clot in 2016.

The 46-year-old marketing director was taking part in a laser combat game at Center Parcs in Woburn Forest when he tripped on some loose wood chippings and needed a plaster cast for three weeks.

During that rest period, a blood clot formed in his calf and travelled into his lung, and Mark was rushed to Watford General Hospital.

Despite treatment at various hospitals, including the New QEII Hospital in Welwyn Garden City, it took more than 18 months for Mark to recover and his lung capacity still works at only 67 per cent.

Mark started swimming to regain fitness, and with an inhaler has now clocked up more than 400 miles and 300 hours in the water.

He was one of only five people to complete all three Great Swim Series in 2018 - that is 10km around Lake Windermere, Alton Water Reservoir and Loch Lomond.

If this year goes swimmingly, Mark is planning on taking on a sea-based challenge for 2020.

He said: "It was such an innocuous and random incident, which unfortunately led to a life-threatening illness. It could happen to anyone but can be easily prevented.

"Unlike heart attacks or strokes, very few people are aware of this. It is sadly little understood by the general public, despite the fact that one in five who experience a PE [pulmonary embolism - lung blood clot] will die with immediate effect."

He has been training five days a week in preparation for the challenge.

Donate at www.justgiving.com/fundraising/mark-smith338.