The black thoroughbred mare, named Roxy, was attacked in her field on Sandy Lane, Tiddington, near Thame in Oxfordshire, on Saturday, 9 February 2008.

There was a three-inch deep triangular cut in her skin, which was hanging away from the bone. The horse's owner, Joanne Connolly, works in the neighbouring veterinary surgery and found Roxy injured at 1.30pm. Just 20 minutes earlier, Roxy had been fine.

An examining vet found that the edges of the wound were very neat and were more likely to be the result of a sharp implement being used rather than it happening accidentally. The horse's head collar had also been moved from one side of the field to the other and it is thought that she may have been held by the collar whilst being cut.

Roxy also received a minor cut to her chin in November last year and another horse, named Tarron, suffered a major wound to his chin. On this occasion, both horses were in their stables on Sandy Lane and were again injured in daylight.

"Roxy is very placid and friendly, especially as she is heavily pregnant, so she tends to be trusting of people and would have proved easy to approach and hold," said Ms Connolly.

"I am now extremely worried about the safety of my horse and her foal which is due in five weeks."

RSPCA Inspector Claire Ryder, who is investigating the incident, said: "The horse was caused a great deal of distress and pain. It is very unlikely that these wounds were accidental and we need to find out who is behind the attacks."

Those found guilty of causing unnecessary suffering to animals could face a maximum six months in prison and/or a £20,000 fine.

• Three British men were to appear in a Stafford court yesterday charged with causing unnecessary suffering to a horse who was attacked by a blade.

The piebald mare, Maggie, was found by her owner with wounds to a hind leg, a cut achilles tendon, a front leg broken above the knee, and cuts all over her body. The injuries to the 19-year-old were too severe and she was euthanised.

The attack happened last April.