A British endurance swimmer has spoken of being protected from a shark by a pod of dolphins as he swam one of the toughest stretches of open water in the world.

Adam Walker is aiming to become the first Briton to complete the Oceans Seven challenge but was alarmed to find he was being accompanied by a shark as he crossed the Cook Strait between the North and South Islands of New Zealand last week.

However, he soon realised he had also been joined on the 16-mile (26km) swim by a pod of dolphins, which appeared to be protecting him.

Mr Walker, who is based in Nottingham, wrote on his Facebook page: "Yesterday I swam the 6th of the 'oceans seven' swims 'Cook Strait' in 8 hours 36 mins.

"But that doesn't tell the whole story rolling waves, big currents but more important than that a dream come true swimming with dolphins over an hour!! I'd like to think they were protecting me and guiding me home!!! This swim will stay with me forever."

After swimming the Cook Strait, English Channel, Strait of Gibraltar, Molokai Channel off the coast of Hawaii, Catalina Channel off California and Japan's Tsugaru Channel, Mr Walker now only has the Irish Channel to go before he has completed his feat, which he was inspired to undertake while watching a film during a flight to Australia.

He is raising money for Whale And Dolphin Conservation.

To sponsor him, visit http://www.justgiving.com/Swim4Whales.