Trainer Archie Alexander has been charged by Racing Victoria stewards after one of his horses returned a swab that breached allowable cobalt levels.

The Alexander-trained Tom's Knight returned a urine sample containing traces of cobalt in excess of the allowed threshold of 100 micrograms per litre.

The swab was taken after the gelding won at Kilmore in November, 2018.

Tom's Knight raced three more times for Alexander before he was transferred to Greg Eurell.

Alexander is facing charges of administration of a prohibited substance and allowing a horse to compete with a banned substance in its system.

The charges will be heard by the Victorian Racing Tribunal at a date to be determined.