Michelle Payne, one of racing’s most high-profile jockeys since she became the first woman to win the Melbourne Cup in November 2015, faces a ban from the saddle of about a month after testing positive for a banned substance while riding at Swan Hill in Victoria two weeks ago.

Traces of phentermine, an appetite suppressant, were found in a urine sample taken from the rider on 11 June, and she will face a disciplinary hearing into the finding on Thursday.

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Reports in Australia suggest that Payne was prescribed the drug by her GP after undergoing surgery to save her pancreas following a serious fall in May last year. However, she is strictly liable for the presence of any prohibited substance in her system and should have been aware that phentermine is banned.

Hugh Bowman, one of Australia’s top jockeys, was suspended for six weeks after testing positive for phentermine in 2003, while Damien Oliver, a three-time Melbourne Cup winner, was banned for four weeks in 2009 after testing positive for ephedrine, another appetite supressant, though the penalty was overturned on appeal.

Payne rode at Royal Ascot for the first time last week aboard the 66-1 outsider Kaspersky, who finished fifth of the 16 runners in the Group One Queen Anne Stakes.

She is also due to be one of the star attractions at the annual Shergar Cup at Ascot on 12 August, as part of a three-strong team of female riders alongside Emma-Jayne Wilson, from Canada, and Josephine Gordon, who was Britain’s champion apprentice last season. A ban of six weeks or longer would rule her out of the event for the second year running, after she was forced to miss both Royal Ascot and the Shergar Cup in 2016 after her fall in May.

Payne, 31, was only the fourth female jockey to compete in the Melbourne Cup in its 155-year history when she won the race aboard the 100-1 outsider Prince of Penzance. In an interview shortly afterwards, she described racing as “a chauvinistic sport”, thanked Prince Of Penzance’s owner for his faith in her, and added that “everyone else can get stuffed [who] think women aren’t good enough” to compete against men.

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It was a performance and victory speech that instantly turned Payne into one of the sport’s major stars, but her attempt to build on her Melbourne Cup success was interrupted by a serious injury in a fall at Mildura racecourse a few months later. Her pancreas was effectively split in two, and surgeons saved the organ only by stitching the larger part to her stomach.

Des O’Keeffe, the chairman of the Australian Jockey Association, told the Australian Broadcasting Corporation’s website on Wednesday that Payne is “extremely disappointed” by the positive test.

“It’s based on the fact that after that split pancreas last year, which was widely reported, her GP has in good faith prescribed her this as something that would assist her,” O’Keeffe said.

“Unfortunately [it is] Michelle’s onus to be aware of what she can and can’t take and that should’ve been provided to the doctor, who then may have come up with an alternative treatment that didn’t breach the rule once tested.

“I think it’s fair to say she’s extremely disappointed that this has occurred. She accepts full responsibility for having this substance, this appetite suppressant in her system.”

Since August 2016, when she successfully applied for a trainer’s licence, Payne has been pursuing a dual career as both jockey and trainer, and took the reins when her first runner finished second in a race at Warrnambool in mid-October. So far, she has trained two winners from 33 runners, with her first success coming at Swan Hill in November.

The story of Payne’s struggle for acceptance as a jockey and her historic success in the Melbourne Cup is also being turned into a film, with Australian actor Rachel Griffiths in the leading role.

Thursday’s tips, by Chris Cook

Newcastle 2.00 Weellan 2.30 Odds On Oli 3.05 Polar Forest (nb) 3.40 Qaffaal 4.15 Symbolic Star 4.50 Circuit 5.20 Safe Voyage 5.55 Pastime

Newmarket 2.10 Roland Rocks 2.40 Theglasgowwarrior 3.15 Mutoodresdashorse 3.50 Saluti 4.25 Khamaary 5.00 Daira Prince (nap) 5.35 Justice Lady

Nottingham 2.20 Agadeer 2.50 Pouvoir Magique 3.25 Zabeel Prince 4.00 Hellomoto 4.35 Artscape 5.10 Shackled N Drawn 5.45 Miningrocks

Newbury 5.50 Diable D’Or 6.20 Queen Of Kalahari 6.50 So Crafty 7.25 Heart Of Gold 8.30 Deeds Not Words 9.00 Monologue

Hamilton 6.00 Tonto’s Spirit 6.30 Quayside 7.00 Wee Jock 7.35 Grey Diamond 8.10 Dark Crystal 8.40 Honeysuckle Lil 9.10 Kenny The Captain