Thompson tested positive for a metabolite of drostanolone (an anabolic steroid) after a pre-season training session, while registered with Whitehaven.
The 23-year-old did not deny or contest the charge before the panel. The National Anti-Doping Panel have found him guilty of an anti-doping rule violation and imposed a two-year period of ineligibility.
His suspension commences from January 30, 2010 which is the day his provisional suspension commenced.
Thompson left St Helens for Widnes at the beginning of 2009, but was soon sent out on loan to Harlequins and then Barrow, before making a permanent move to Cumbria with Whitehaven.