Leith Hill tower in Surrey, a Georgian folly, is the highest point in south-east England. Through its telescope visitors can see Big Ben to the north and the glimmer of the Channel in the south. With its surrounding wooded hillside, it ought to be a tranquil rural idyll.

It has, however, become a battleground between horse riders and cyclists who have clashed over a new mountain bike trail that in parts runs alongside a bridleway. The dispute has meant that the trail has been closed while claims by the British Horse Society that it is illegal are investigated. The society also claims that the proximity of fast-moving bikes to horses will result in a fatal accident.

In the New Forest, walkers and riders are free to go where they like while bikes are restricted to gravel paths. But despite the prospect of a £500 fine, it is claimed, many mountain bikers break the rules. Last September David Horton, 64, died after he fell from his horse as it was spooked by a cyclist coming up at speed behind him on an unauthorised route near his home in Beaulieu.

And in Delamere Forest, Cheshire, disputes between the two groups have resulted in a trial riding code for mountain bikers urging them to be careful around horses and to warn riders that they are approaching. However, riders claim their horses are often startled by cyclists doing jumps and fast steep descents across paths that they use.

Bob Milton, common land adviser to the society, has written to Surrey county council, which manages Leith Hill on behalf of the National Trust, saying that the new track is not lawful, as cyclists do not enjoy the same access to common land as riders and walkers. "There have already been a number of accidents caused by speeding and aggressive mountain bikers with equestrians and pedestrians, though no biker has remained in place for long enough to be asked for their name," he said.

The decision to close the track was, he said, "welcome, but does show what a complete lack of understanding the council has of the legalities and ramifications of an access agreement under the National Parks and access to the Countryside Act 1949. It is the prerogative and duty of the council as scheme managers to act only under the agreement with respect to public access which excludes vehicles of any sort, ie cycles."

Tessa Gooding, a local rider, said of the new track: "Of most concern is a 100ft blindspot where cyclists won't know if there is a horse in the sunken bridleway beneath them. I have this terrible image of my young racehorse bolting into a family as a result of being spooked." Penny Tyson-Davies, BHS bridleways officer for Mole Valley, said there had been no input from equestrians into the building of the trail. "Mountain bikes whizzing in and out of trees, jumping ramps above horses' heads, around an established sunken horse track, is an accident waiting to happen."

Surrey county council said it was waiting for Milton to confirm whether he wanted the council to investigate his complaint. If so, it would cooperate fully.

Julie Rand, from the national cycling charity CTC, said: "There are bridleways and tracks all over Surrey that are quite happily coexisting at the moment without too much anguish and they respect each other. People are anticipating problems that may not actually arise." Sam Bayley, National Trust head ranger, said: "The design will ensure cyclists naturally slow down at crossing points by appropriate turns and signage."

Rob Fairbanks, of the Surrey Hills Area of Outstanding Natural Beauty Board, said: "It is not feasible to ban biking in one of the most popular areas in England. We want to work with the BHS to educate cyclists about the priority that needs to be given to horse riders."