RSPCA inspectors spotted the elderly grey mare, called Hannah, in a field near Houghton.

Ribs were protruding through its dirty coat, which was covered in lice.

Officers took the animal to an RSPCA shelter, where it was found to weigh just 350 kilos – about 70 kilos below its normal size – and have badly overgrown teeth.

Vets said the 32-year-old animal had probably been suffering for "months and years rather than days and weeks".

Owner Heather Bell, who was on holiday at the time, was later interviewed by a RSPCA inspector.

Sunderland Magistrates' Court heard yesterday that Bell, of Lady Beatrice Terrace, New Herrington, had refused to sign the horse over to the animal charity.

Prosecuting for the RSPCA, Denise Jackman said the mare was still being cared for at the shelter, and now weighed more than 430 kilos.

Phil Dyer, defending, said 48-year-old Bell had owned Hannah since she was seven.

He described the case as a "tragic misunderstanding", saying Bell thought the animal had been losing weight because of its age but was afraid to call a vet in case the horse had to be put down.

"She is a caring, dedicated horse owner who made a mistake," Mr Dyer said.

"Before she went on holiday for a well-deserved break she should have called the vet instead of thinking 'I will wait until I get back'."

Bell pleaded guilty to causing unnecessary suffering to an animal.

Magistrates imposed a £440 fine, and ordered she pay £450 towards the RSPCA's costs, £1,000 towards boarding fees, £150 court costs and a £15 victim surcharge.

She was banned from keeping a horse for 10 years and was ordered not to appeal the ban for at least five years.

Ownership of the horse was handed over to the RSPCA and vets' and photography fees were paid out of central funds.

Speaking after the hearing, Inspector Emma Ellis said: "This case highlights that even if an animal is elderly it does not take away the need for veterinary treatment – in fact it is even more important."