The citing complaint made by the Citing Commissioner for the match, Murray Whyte (Ireland), was for acts contrary to good sportsmanship in contravention of law 10.4(m) in that Rees had made contact with the eye/eye area of Saints hooker Dylan Hartley 30 minutes into the first half.
Rees pleaded not guilty to the complaint on the basis that any contact had been accidental.
The Judicial Officer determined that the 27-year-old was guilty of foul play in that he had made contact with the eye/eye area of England international Hartley. The Judicial Officer found that the contact had been reckless but not intentional.
Rees' actions were deemed to be at the low end of the level of seriousness for an offence of this type, and having taken into account all aggravating and mitigating factors imposed a suspension of 12 weeks. Rees is free to play again on March 31, 2011.
Both parties have the right to appeal.
The current length of suspension rule Rees out of this year's Six Nations, which begins when Wales welcome England to the Millennium Stadium on Friday, February 4.