ฉัMichael Perham, who became the youngest person to cross the Atlantic alone when he was 14, left Portsmouth at 11:10 am aboard a 50-foot (15-metre) yacht, his spokesman Kizzi Nkwoch told AFP.
He will cover 21,600 nautical miles, crossing every single line of longitude and the equator, in four and a half months, and his only contact with family members back home will be limited to brief satellite conversations.
Perham is expected back in Portsmouth in March 2009, around his 17th birthday.
"I'm a little bit nervous but otherwise really, really excited," Perham said before setting sail, admitting that the voyage was "a little crazy".
"It's just the feeling of being completely in control, relaxed and at one with nature. It's just fantastic. But you don't look forward to the fact you are alone for about four months."
The youngster started sailing when he was aged seven and was initially inspired to break the record for crossing the Atlantic after Sebastian Clover, aged 15, sailed from the Canary Islands to Antigua in 2003.
After managing to cross the Atlantic in six weeks, setting off from Gibraltar in mid-November 2006 and arriving in Antigua in January, he immediately began planning to break the round-the-world record as well.
The current record holder is Australian Jesse Martin, who set the feat aged 18 in 1999.
Perham, who will be entirely unassisted in his journey, will sail along the African coast before crossing the Pacific and Southern Oceans via the Cape of Good Hope in South Africa, Cape Leeuwin in Australia, and Cape Horn in South America, according to his website.