The ex-Hull KR prop had only recently joined the West Yorkshire club from St Helens when he failed the test following their win at Widnes Vikings on July 14 last year.
Walker was suspended by the Rugby Football League the following month and banned from all playing and training activity.
He parted company from Trinity and the 27-year-old has now been notified of his official ban from UK Anti-Doping. Walker received a reduced suspension after proving he was suffering from depression and anxiety at the time. He can resume playing on March 14.
UKAD chief executive, Nicole Sapstead, said: “While UKAD accepts ADRVs (anti-doping rule violations) don’t always stem from a deliberate intention to cheat, athletes must always adhere to the principle of strict liability.
“Cocaine is banned in-competition and athletes are solely responsible for what is in their system, regardless of whether there is an intention to cheat or not. Sportspeople must be aware using cocaine, even out-of-competition, will put them at risk of breaking the anti-doping rules and receiving a long ban.”
Widnes’ ex-England stand-off Rangi Chase was also tested positive after the same game and received a two-year ban while Castleford’s Zak Hardaker was suspended for 14 months,
The UKAD report revealed Walker produced medical evidence from a pychiatrist to the effect that he was suffering from marked difficulties with anxiety and depression.
It read: “He states that he had consumed cocaine because a difficult set of personal circumstances led him to make efforts to ‘escape’ his difficulties.
“...this use of cocaine at the time can be understood in the context of him self-medicating these mental health problems whilst at the same time also having developed a psychological addiction to cocaine.
“UKAD is satisfied that the mental health difficulties that Mr Walker suffered at the time are specific and relevant to the assessment of his level of subjective fault.”