Champion jockey Oisin Murphy has apologised to his fans after failing a breath test at Newmarket races.
The 26-year-old, currently leading the jockeys' title race, was stood down by the racecourse's medical officer.
He was due to partner Mise En Scene in the Group 1 bet365 Fillies' Mile on the card – the top level of the sport – for main employers Qatar Racing.
Murphy told his 48,000 Twitter followers that he would not be able to take any part in Friday's fixture.
He said: "I was requested to take a breath test at Newmarket today. I blew under the drink drive limit but over the riding limit. I’m sorry to all the people I’ve let down today. I take full responsibility. I will be riding tomorrow."
Murphy's first ride of the day was due to be the George Boughey-trained Fiorina in the 7f maiden.
A statement from the British Horseracing Authority said: "An inquiry was held following a report from the Equine Welfare Integrity/Collection Officer that Oisin Murphy, who was declared to ride Fiorina, had failed a breathalyzer test prior to the race.
"Murphy, the Equine Welfare Integrity/Collection Officer, and Raceday Assistant were interviewed. Having heard their evidence, the matter was referred to the BHA for further consideration, and Murphy was stood down for the remainder of the day."
The alcohol limit for jockeys is 17 micrograms per 100ml of breath, while the UK driving limit is 35 micrograms per 100ml.
At the top of the 2021 Flat jockeys' championship, Murphy, who has won nearly £4million of prize money for the whole year, is 11 winners clear from William Buick.
It is due to conclude on October 16, coinciding with the season's grand finale, British Champions Day at Ascot.
Murphy is due to ride one of the favourites Buzz in Saturday's feature at Flat racing's HQ, the Together For Racing International Cesarewitch.
He has 147 winners on the board for the current campaign, which has included two Group 1 victories on the Andrew Balding-trained Alcohol Free.
Murphy, a former champion apprentice in the saddle, was also crowned top jockey at Royal Ascot for the first time.
