The extreme form of triathlon in the American state will see athletes begin with a 2.4-mile ocean swim in Kailua Bay.

This is followed by a 112-mile bicycle ride along the Queen Ka'ahumanu Highway from Kailua-Kona to Hawi, before a marathon run.

Home United States hopes rest on Heather Jackson, who last year became the first American woman to reach the Ironman World Championship podium since 2006.

She earned bronze in a time of 9 hours 11min 32sec.

This season, she collected wins at Ironman races in Peru, Canada and the US, and finished second to Great Britain's Rachel Joyce in Boulder.

However, Switzerland's defending women's champion Daniela Ryf will be a keen rival.

She won last year in 8:46:46 and is looking to clinch a third consecutive title. 

At the top of the start list for the men's race are the Germans who swept the top three places in 2016.

They are defending champion Jan Frodeno, runner-up Sebastian Keinle and third-place finisher Patrick Lange.

Ben Hoffman was the top US finisher last year, with a time of 8:13:00, and his achievements in 2017 include a third-place at the Ironman event in Boulder and a runner-up spot in Santa Cruz.

Fellow American Tim O'Donnell is also a top contender in the men's field, having finished on the podium in his last five races.

Matt Hanson has also had an impressive season, winning the Ironman North American Championship in Texas in April as well as the Ironman 70.3 Couer d'Alene in June.

Tim Don was due to have been one of the British representatives, but will miss the event after suffering a neck fracture in a collision with a car while riding his bicycle near Kona Airport.

Don was 10th in the first Olympic triathlon in Sydney in 2000 and 18th at Athens in 2004. 

He won the 2007 London Triathlon and set a new Ironman world record of 7:40:23 at the Ironman South American Championship in Florianopolis, Brazil in May. 

He suffered a fracture in his C2 vertebrae.

"The good news is I don't need an operation," he wrote on his Instagram account. 

"The bad news is I'm out for the best part of five or six weeks in a brace."

Ironman, now a Wanda Sports Holdings company, has been organising the event since 1978 and this year's edition will see athletes from 66 countries, ranging in ages from 18 to 84 across different category races.

A total of 39 per cent come from Europe, 37 per cent from North America, 17 per cent from Asia and Oceania and five per cent from South America.

"From the most modest of beginnings, the Ironman World Championship has evolved into an extraordinary international event with more than 2,400 registered competitors from 66 countries around the world," Andrew Messick, Wanda Sports Holdings' chief executive, said.

"These athletes have earned their slots in Kailua-Kona through extraordinary feats and deserve the right to race with the world's elite."

The most represented country is the US, with 741 competitors from 48 of the 50 states, followed by Australia with 234, Germany with 217 and Great Britain with 150.