A 70-year-old grandfather today became the oldest person to swim across the English Channel, making it to France after a marathon slog of almost 18 hours.

Roger Allsopp waded into the chilly sea at Shakespeare Beach in Dover, southeast England, at 8:00 am (0700 GMT) on Tuesday, aiming 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, was accompanied by a support boat to help him navigate across one of the world's busiest shipping lanes.

But as he battled against tides and currents, he took a zig-zag route to reach the continent, swimming for 17 hours and 51 minutes before reaching the shore near Wissant in the early hours of Wednesday.

"I do feel an immense sense of achievement and relief that I have been successful," said Allsopp, who is 70 years and four months old.

"This has been an incredible personal challenge for me.

"That a man of my grand age can achieve such a physical and mental challenge proves that you can live younger if you keep active in mind and body."

Allsopp was raising money to fund medical equipment for cancer research at the University of Southampton, southern England.

Official Guinness World Records adjudicator Anna Orford said: "To swim the English Channel is a great challenge and to do it at the mature age of 70 years is astounding.

"We are very pleased to congratulate Roger on this achievement and in doing so setting a new Guinness World Record."

The previous record-holder was 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.