A post-mortem examination has revealed the sudden death of Many Clouds at Cheltenham was caused by a severe pulmonary haemorrhage which could not have been foreseen.

The Hennessy Gold Cup and Grand National winner had just beaten Thistlecrack to claim a second victory in the Cotswold Chase on Saturday when he collapsed shortly after the line.

Although the horse had suffered post-race ataxia in the past, which had resulted in over-heating and wobbling on his feet, Tony Welsh, acting chief veterinary officer for the British Horseracing Authority, said those symptoms "were in no way present or associated with his sad death at Cheltenham".

Pulmonary haemorrhage through exercise occurs because of a broken blood vessel.

No significant underlying health issues were discovered in the autopsy, the BHA said, adding that Many Clouds had worn a heart-rate monitor in training on November 17, 2016 and the data collected on that occasion was within normal parameters in relation to both his heart rate during exercise and his recovery rate after exercise.

"Episodes such as this are rare, and can occur in horses which have no underlying health issues, and amongst all disciplines of sport horses," Welsh said.

"In spite of the rarity of these incidents, as a sport we are determined to do more to understand what causes these symptoms, and whether more can be done to prevent it.

"Post-race ataxia and similar symptoms are linked to an increase in body temperature after exercise and can be treated by providing the horse with water. It is not uncommon in racehorses or other sport horses.

"Despite some reports following the incident, there is no existing veterinary evidence which links these symptoms with racehorse fatalities, and the post-mortem results have categorically proved that the symptoms exhibited by Many Clouds in the past were in no way present or associated with his sad death at Cheltenham."