Knights star Jarrod Mullen in steroids scandal

The Newcastle Knights have been left "disappointed" after Jarrod Mullen was found to be in breach of the NRL's anti-doping regulations.

The club released a statement explaining that the Knights star had returned a positive sample for an anabolic steroid.

He could face a ban of up to four years if found guilty.

Mullen was tested by the Australian Sports Anti-Doping Authority (ASADA) on November 29, 2016.

"The club is obviously extremely disappointed," Knights CEO Matt Gidley said.

"The club has a strict governance program, the players are regularly educated and fully aware of the consequences of going outside our governance guidelines.

"From here, there is a formal process we need to follow under the guidelines.

"We need to respect the process and in the interim the club will continue to monitor Jarrod’s welfare."

Under the Anti-Doping Policy, Mullen now has the opportunity to have his B-sample analysed.