Shelly Pritchard and Barrie Jones of Carnsdale Show Horses bought DJP Royal Heir as a three-year-old and re-passported him as Carnsdale Showtime before selling him on.
But breeder Daniel Park realised what had happened and took it up with the NPS.
The second passport has now been revoked and the horse’s name reverted.
Mr Park told H&H: “You don’t make money as a breeder. He’s a lovely horse and I want him to carry my prefix, that’s my recognition.
“When I sold him he had a NPS passport, but when he was sold by Shelly it was with a Horse Passport Agency passport.”
NPS chief executive Peter Durrant said: “We would only change the name with the agreement of all parties. Owners have a responsibility to understand passport legislation.”
Ms Pritchard told H&H she often changed horses’ names to her prefix, but did not usually apply for a new passport. She said she was unaware that applying for a second passport was against legislation.
Ms Pritchard added: “I usually buy from Ireland and they are all microchipped, so I pay the Irish Horse Board to change the name.
“This was the only horse I bought in the UK this year and as it did not have a microchip I thought I could get it a new passport. I did not realise I was doing something wrong.”
The confusion came to light after the horse was sent to Jerome Harforth to produce. Mr Harforth recognised the breeding and contacted Mr Park.
“I was lucky,” added Mr Park. “Most breeders would not know it was their horse coming out under saddle as they’d last seen it as a foal.”
This news story was first published in the current issue of Horse and Hound (3 November, 2011)