A 57-year-old man is aiming to swim from England to France for the 34th time on Friday, and seize the title of "King" of the English Channel.
David Walliams took advice from Mr Murphy on his Channel swim
Kevin Murphy, president of the Channel Swimming and Piloting Federation, who set out at 0734 BST, said he wanted to take the title "once and for all".
Mr Murphy's nearest rival, Michael Read, has also completed the 21-mile swim 33 times.
Mr Read is chairman of the Channel Swimming Association.
IRN broadcast journalist Mr Murphy first swam the Channel in 1968.
He went on to swim around the Isle of Wight and along the River Thames from Richmond to Gravesend, Kent.
Mr Murphy also completed three non-stop crossings from England to France and back again.
And he advised comic David Walliams on his training for his Sport Relief swim across the Channel earlier this month.
High winds prevented Mr Murphy setting off on Thursday, but the attempt got under way on Friday morning, his spokesman John Nolan said.
Before the attempt, Mr Murphy said: "It never gets any easier but I've trained hard and I'm very determined to make the crossing and take the King of the Channel title once and for all."
Speaking about his rivalry with Mr Read, who is chairman of the Channel Swimming Association, he added: "He won't say, but the word is he is going to have a go, although not until September.
"If he keeps going, so will I - and he's older than me."