The verdicts in past British Horseracing Authority cases were called into question on Thursday when the chairman of a disciplinary panel suggested previous panels may have misdirected themselves about the test to apply when considering the penalty to be imposed on a trainer over a failed dope test.

Stuart Morrison, a retired solicitor and former immigration judge whose colours were carried to big-race glory by Quick Ransom and Bijou D’Inde 20 years ago, made the comment in a hearing about the Oaks-day success of De Bruyne Horse, who has now been disqualified.

Richard Hannon, the trainer of De Bruyne Horse, was fined £8,000 because the colt tested positive for a metabolite of tramadol after winning the Woodcote Stakes at Epsom in early June. It is the third time the trainer has been called to the BHA over tramadol positives, involving four horses from his Wiltshire yard over the past three years.

Hannon expressed himself baffled as to the cause, which a detailed BHA investigation also failed to discover. The trainer said he had spent thousands of pounds since June in trying to prevent another such positive, on random tests, on the installation of many hand sanitisers and in changes to his staff contracts underlining the importance of observing stable rules.

BHA rules allow a trainer to escape punishment if they have taken all reasonable precautions to prevent a positive test and if the panel is persuaded the substance was not administered intentionally by anyone. Ultimately, Hannon failed the first of those two tests, the panel deciding he could have acted sooner to take the precautions he has taken since June.

However, Morrison clashed with Tim Naylor, the BHA’s head of regulation, over the question of intentional administration. Morrison noted the verdicts in the two previous cases involving Hannon declared: “The panel was unable to establish the source of the substance and could not therefore be satisfied that the administration of the substance was accidental.”

Morrison, referring to the recent appeal board verdict in the Philip Hobbs case, said: “As you know, Mr Naylor, that is not the test.” That ruling says a panel can review the evidence and decide, on the balance of probabilities, that there was no intentional administration, even where the question of the source remains a mystery, just as Morrison’s panel eventually decided in this case.

Naylor said he did not accept the previous panels had erred. “It does not follow that, because the BHA does not establish the source, automatically the source must be accidental,” he said. He suggested those previous panels had made their decision on a review of the evidence in those cases.

If Morrison is right, many trainers may have been punished unfairly by past panels, assuming they could show they had taken all reasonable precautions to prevent a positive test. The point is likely to come up again at the imminent hearing involving the trainer Hughie Morrison, one of whose horses tested positive in January for an anabolic steroid, the source of which remains unknown.

Hannon’s £8,000 fine is only a limited increase on the £5,000 fine he received for his previous tramadol positive last year. He will take time to consider whether to appeal.

“The hearing was fair and the panel was fair,” he said. It’s unfortunate the fine was heavy but it’s impossible to prove where it came from. Personally, I’m satisfied it was not administered on purpose. Identifying the source in any yard is nearly impossible, let alone a yard that sends out 1,600 runners a year and employs 90 full-time staff across two sites, miles apart.

“I’ve tried to do everything in my power to prevent it happening again. However, the percentage of it happening since I started training is 0.073% [per runner]. Which is minimal. That’s why we didn’t do anything after the first one. We just thought it was a freak incident.

“We’re not police constables, we’re not evidence gatherers. We’ve got a massive industry to run. We’ve got to look after the lads, the horses, the owners, we’ve got entries … the business is massive. We’ve got to worry about the hay, the straw, the feed coming in. We’ve got gallops to maintain. There’s so many things. And then things turn up like this. I appreciate they’re important but they’re freak incidents when you look at how often they occur. We have security cameras everywhere.”

Hannon told the panel he had three members of staff “of a medication age” who might have been taking a painkiller such as tramadol. All have since left his employ for unrelated reasons. While he has asked his staff to notify him of any medication they are taking and some have responded, he does not believe he has the power to command them to hand over medical records, a point with which Morrison agreed.

“We’ve covered as much ground as we can in trying to avoid another positive at all costs,” Hannon said. “We’ve made our staff aware of that. There are hand sanitisers everywhere, including at the new clock-in system and where they make their tea and coffee. I still don’t know if that will stop it.”

At a cost of £2,000 per time, Hannon has hired a vet he does not otherwise use to choose 20 horses at random and test them for banned substances. He intends to continue doing that at intervals. So far, no such test has shown up tramadol and nor did any of the BHA tests when it conducted unannounced testing at the stable in the week before Royal Ascot.

After learning of the Epsom positive, Hannon elected to have De Bruyne Horse tested before he ran in the Coventry Stakes at Royal Ascot two weeks later, finishing eighth of 18, beaten less than three lengths. In the event, the result was not available before the race but it ultimately returned negative.

Hannon suggested that dogs, sometimes treated with tramadol, could be a source of the problem when they urinate onto horses’ bedding and reported he had banned dogs from his stable yards since this latest positive. But he said it was a difficult ban to enforce, with staff living on the site and owning their own dogs, while service providers sometimes bring their dogs on to the site.