Endurance swimmer Lewis Pugh has announced plans to swim the length of the English Channel, to urge the Government to better protect UK waters.
Mr Pugh will attempt to swim 560km (350 miles) along the English coastline from Land's End in Cornwall to Dover, Kent, in just Speedo swimming trunks, a cap and goggles, in what he said would be his "toughest" swim yet.
"The Long Swim", which begins on July 12, marks the start of a worldwide Action for Oceans campaign calling on governments to fully protect at least 30% of the world's oceans by 2030.
Mr Pugh is calling on the UK Government to urgently strengthen marine protected areas around the UK and its Overseas Territories.
He warned just seven square kilometres (2.7 square miles) of the 750,000 square kilometres (290,000 square miles) of UK coastal waters are currently fully protected, a figure he said was "shocking".
The campaigner and UN Patron of the Oceans also said the route would allow him to meet people along the way for beach cleans, as he warned of the need to stop plastic from entering the rivers and seas.
As well as taking part in the beach cleans organised by marine conservation charity Surfers Against Sewage, people are also being invited to join him for stretches of the swim and be "a voice for clean and healthy seas".
The Channel Swimming Association, which has been authenticating cross-Channel swims since 1927, will observe and verify his swim, which will see him average five hours and 10 to 20km (six to 12 miles) each day.
Mr Pugh will be making the first attempt to swim the length of the Channel according to the Association's rules, without artificial aids and just wearing swimming trunks, cap and goggles.
Mr Pugh has previously used "Speedo diplomacy" to help create the world's largest marine protected area in the Ross Sea, Antarctica, and has also swum in the Arctic and in the icy waters of South Georgia in the south Atlantic as part of his efforts to raise environmental awareness and protect nature.
He said: "I've been swimming in the world's oceans for 30 years. This is not a long time in ecological terms, and yet I've seen the oceans change before my eyes.
"I am swimming the length of the English Channel to call on the British government to urgently protect the waters that surround the UK."
He called for the introduction of marine protected areas (MPAs) where wildlife and habitats are fully protected from human activity.
"It has been shown that fully protected marine protected areas give distressed and degraded seas their best chance of recovery.
"But the need for action is urgent, and the time to act is now. In a few years' time, it will be too late to fix this crisis," he said.
The British swimmer has been training in the cold, rough waters off South Africa and the Falkland Islands, swimming an average of seven kilometres (four miles) a day to prepare for the Channel swim.
He has put on weight to prepare for the challenge and believes exhaustion and repeated exposure to cold will be his biggest problems, while the Channel is also "the world's busiest shipping lane, and has plenty of jellyfish".
He added: "There will be bad-weather days when I simply won't be able to swim.
"The English Channel also has very strong tides. I've got to get the timing right, or I could be going backwards."