Some riders were unable to compete as a consequence.
Gloucestershire police said nine lorries were plundered during the the break-ins, which happened on the gala night, Friday, 13 April.
It is the second year running that thieves have targeted boxes at the show.
Brian Frost, whose son Jordan was competing, lost almost £1,500 worth of equipment from his locked lorry, including a Whitaker saddle, stirrups, stud girth and a wheelbarrow.â€¨
“I lost less than others, but I’ve taken an issue with [the organisers] Show Direct,” he told H&H.
“The gates were left open and the lights were turned off at 11pm. It’s a wonder more tack wasn’t stolen.
“There was the least security I’ve ever seen, even at some of the smaller shows – it was very disappointing.
“My son missed out competing on the Sunday – we had to go home early as we didn’t have the tack and didn’t feel safe,” he added.â€¨
Bobbie Martin-Pope, who was grooming at the championships, agreed the show needed better security.
“People were checking their tack lockers and finding everything gone. I was astounded, especially after last year,” she said.
“It was all the expensive tack that was taken – Whitaker and Prestige saddles. It’s the worst show I’ve been to, security-wise,” she added.
But Kelvin Bywater from Show Direct said security had been increased since last year.
“There was a theft in 2011 that involved tack being stolen from three horseboxes during the Friday gala evening, so additional security measures were taken this year,” he said, but declined to say what.
“This is a reminder to all to ensure equipment is insured and kept secure,” he added.
Anyone with any information about the thefts is asked to contact Gloucestershire police, tel: 01452 728199.
This news story was first published in the current issue of H&H (26 April 2012)
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