A third horse from the Lee and Shannon Hope stable has returned a positive test to cobalt.

Racing Victoria stewards confirmed on Thursday a positive swab was returned by Choose in a pre-race urine sample at Caulfield on September 28 before the mare ran unplaced.

Stewards said the sample contained cobalt at a concentration in excess of 200 micrograms per litre in urine which is prohibited under the rules of racing.

Cobalt is an essential mineral nutrient which affects multiple body systems and is toxic at high dosages.

Stewards are continuing investigations into the pre-race urine samples taken from Windy Citi Bear at Geelong on June 25 and Best Suggestion at Caulfield on July 5 which also detected cobalt above the allowed threshold.