Sir Ranulph Fiennes and Dr Michael Stroud are risking permanent damage and even death by trying to complete their marathon challenge, a sports scientist has warned.

The men could suffer potentially fatal kidney failure or develop early-onset arthritis because their bodies will have no time to recover between each of their 26-mile runs, due to be completed tomorrow.

Professor Clyde Williams, professor of sports science at Loughborough University, said: "They are really punishing their bodies, possibly to the point of death. We could never get approval from an ethics committee to conduct an experiment on people like this. They are both obviously very mentally strong, but their bodies may not bounce back as quickly."

While running, the body uses up glycogen, a type of sugar found in the muscle. Only limited amounts of glycogen can be stored and, once those supplies are used up, the body begins to burn fat for energy. After running 26 miles, people normally eat and drink enough to rehydrate and replenish their glycogen stores, but Sir Ranulph and Dr Stroud will not have enough time to do this before starting their next marathon.

Pounding the race routes could do serious damage to their knees and hips, but tiny tears in their muscles could also cause leaks of a molecule called myoglobin. If that leaks from the muscle tissue, it can block the filtering function of the kidneys and cause a potentially fatal condition called exertional rhabdomyolysis.