A 70-year-old retired breast cancer surgeon hopes to become the oldest person to swim the English Channel.
Roger Allsopp set off from Shakespeare Beach in Dover, Kent, this morning and aims to reach Cap Gris Nez in northern France within 16 hours after several days of postponements due to the weather.
Aged 70 years and four months, he hopes to beat the current record holder, George Brunstad, the uncle of Hollywood star Matt Damon.
Retired American Airlines captain Mr Brunstad, from Connecticut, swam the Channel aged 70 years and four days in 15 hours and 59 minutes on August 29 2004.
Before he made final preparations in Dover, Mr Allsopp, from Guernsey, said: "I'm having mixed feelings because I have been hanging around for so long, but I'm very keen to get going."
Mr Allsopp will be backed by a support crew as he makes the punishing 21 nautical mile crossing to Cap Gris Nez, the closest point of France to the UK.
He has been gearing up for the challenge after being inspired by an inscription at a pub in Dover marking Mr Brunstad's cross-Channel achievement.
His attempt comes 136 years after British merchant navy captain Matthew Webb became the first person to swim the Channel, doing the breaststroke from Dover to Cap Gris Nez in 21 hours, 45 minutes.
Mr Allsopp is motivated by a personal crusade to raise £750,000 for world-class medical equipment to help advance cancer research at the University of Southampton.
The money would pay for new equipment to analyse the blood of cancer patients and non-cancer patients to develop a test that would give a pre-warning of cancer.
He has received a personal pledge of £250,000 from Derek Coates, the chairman of nutritional and well-being supplement company Healthspan.
Grandfather-of-three Mr Allsopp, the BBC's Guernsey Sports Personality of the Year winner in 2006, said: "I've never been one to sit on my laurels.
"Retirement has been wonderful but I know how important it is to keep active in mind and body to live a healthy retirement and so I set myself physical challenges to ensure that I keep as fit and healthy as I can."
Mr Allsopp has a record of physical challenges having run the London Marathon aged 60 and completed his first Channel swim at the age of 65 in 2006, at the time becoming the oldest Briton to do so.
Craig Glenday, editor-in-chief of Guinness World Records, said: "To swim the English Channel is one thing, but to do this at the mature age of 70 years is astounding.
"We hope to verify this as a Guinness World Records achievement as soon as Roger completes the feat."
Progress of Mr Allsopp's swim can be followed at http://www.healthspan.co.uk