Brisbane Lions defender Justin Clarke has been ruled out of football indefinitely due to concussion. 

Clarke suffered a head knock at training on January 18, falling to the ground heavily after a pack mark contest.

He was taken to hospital where scans revealed no break or fracture to his neck, and he was discharged from hospital the same night.

However, the 22-year-old has not returned to training since the incident, and has undergone regular concussion testing in line with the AFL's protocol.

Justin's symptoms have persisted and Lions medical staff have consulted concussion specialists in Brisbane and Melbourne.

Testing has not shown any damage to his brain but with symptoms continuing, the Club and Justin will take an ultra-cautious approach.

Lions Football Manager Matt Francis said the Club's overriding priority is Justin's long-term health.

"Justin has had concussions before and our duty of care to him is paramount," said Francis.

"Our medical staff and Justin are working through his symptoms on a daily basis and there is no timeframe set for his return.

"Justin has been completing a university degree during his time with the Club and he has very definite plans for his career post-footy - that will influence any decisions going forward."

The Club will provide updates on Justin's condition in coming weeks.