Trailwalker, the 100km trek organised by Oxfam, the Queens Gurkha Signals and The Gurkha Welfare Trust has been cancelled due to the unusually hot weather.
The annual trek across the South Downs Way was scheduled to take place this weekend.
With temperatures set to rise to more than 27degC, the Army has advised that participants are at risk of heat injury and exhaustion if the event goes ahead as planned.
Teams made up of four walkers with a two-person support crew must complete the distance in 30 hours.
Matt Jerwood, Oxfam Head of Events, Festivals & Regional Engagement said: “The organisations which run the event have reluctantly taken the decision to cancel Trailwalker 2018 after receiving expert advice from the Army.
“I am devastated for our inspiring supporters, who have trained so hard to take part and raise money for Oxfam and The Gurkha Welfare Trust’s vital work.
“But the health and well-being of the participants has to be the absolute priority. The expert advice we have been given is the predicted hot temperatures and dry conditions present a real risk to walkers, so of course we must cancel.
“We had hoped the weather might change but specialist forecasting from the Met office, received this morning, made it clear that it is going to be dangerously hot.”
Entrants will be offered the chance to defer to 2019.
Alternatives were explored, including a shorter route and alternative timing, but a viable option could not be found to run Trailwalker without a risk to either safety or the logistical operation.
Two thousand participants registered to take part in this weekend’s event. They tackle the trek in teams of four, walking alongside the Queens Gurkha Signals who complete the exercise as part of their military training. The event takes one year to plan, with careful coordination of logistics and supplies.
Trailwalker has been held for the past 20 years and the money raised is shared between The Gurkha Welfare Trust and Oxfam.