Jock Paget says he is “absolutely dumbfounded” over the discovery that his horse, Clifton Promise, tested positive for a banned substance at Burghley last month (8-11 September).

The horse tested positive for the substance reserpine — a powerful tranquillizer used to treat humans which is on the FEI banned substances list. Banned substances are those deemed by the FEI to have no place in equine medicine.

“It’s been a massive shock to us, devastating news,” Jock told Fairfax Media.

“We are looking at everything and trying to find out just how this has happened. There’s obviously been some sort of mistake, but I’ve got no idea at this stage.”

The FEI will now test the “B” sample to determine if the first result was correct, with the results due in 2-3 weeks time. If the “B” sample is also positive then Jock will lose his Burghley title and could face a lengthy ban.

He is currently suspended pending the outcome of the “B” sample test — which means he will not be able to compete at Pau (23-27 October), the last four-star horse trials of the year.

Jock has reportedly requested that the “B” test is sampled in a different laboratory to the “A” sample.

“I haven’t even heard of it (reserpine), I have never done anything with that sort of stuff. It’d be sheer stupidity to because you know if you do well your horses will be tested, I’m absolutely dumbfounded by it all,” Jock added.

“To my knowledge, from what I’ve heard in the past 12 hours, you can’t even get this stuff in the UK although I’m not 100 per cent sure on that. I have also learned it can be confused with other substances and plants so we need to find out more about that.

“The whole thing is just crazy.”

Clifton Pinot, the horse ridden by Jock’s mentor — Australia’s Kevin McNab — also tested positive for the same substance at Burghley.

Kevin told H&H that he had “no comment” on the positive test.

Frances Stead of Clifton Eventers owns Clifton Promise and also joint owns Clifton Pinot with rider Kevin.

A statement on Clifton Eventers website said: “Clifton Eventers is deeply shocked by these events and reiterates that all the relevant parties state categorically that none of them have taken any actions with the intent of administering any prohibited substance.

“We are working with both the respective National Federations on establishing the probable cause of these testing results.”