The Ron Quinton-trained Boss Lane has been confirmed with an illegal cobalt level in a sample taken before he ran fourth in the Missile Stakes.

Former champion jockey Quinton, who is one of Sydney's most respected trainers, has had three runners since August 16 while Racing NSW stewards investigated his stable.

On Wednesday, stewards received confirmation Boss Lane's cobalt level was above the 100mcg/L allowable threshold in the sample taken before he ran in the Missile Stakes at Randwick on August 5.

During the investigation, stewards took 35 out-of-competition samples from horses in the Quinton stable.

Grafton trainer Gordon Yorke will also front an inquiry into a high cobalt level returned by Follow Through in a sample taken before he ran second on his home track on July 13.

Dates for the hearings have yet to be set.