A 70-year-old grandfather waded into the chilly sea on Tuesday in a bid to become the oldest person to swim across the English Channel.

Roger Allsopp set off from Shakespeare Beach in Dover at 8:00am, heading for Cap Gris-Nez on the northeast French coast, some 21 miles (34 kilometres) away.

The retired breast cancer surgeon, from the Channel Island of Guernsey, is accompanied by a support boat to help him navigate across one of the world's busiest shipping lanes.

"I've never been one to sit on my laurels," said the grandfather of three, who is aged 70 years and four months.

"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."

The current record-holder is retired airline pilot George Brunstad -- the uncle of Hollywood film star Matt Damon.

The American swam the Channel aged 70 years and four days in August 2004.