The 27-year-old scrum-half, who has featured in 42 Magners League matches since his debut in the 2007/08 season, was cited following his side's second Heineken Cup encounter with Northampton last month.
Rees's case was heard at a disciplinary hearing on Thursday afternoon at which he was found guilty of making contact with the eye/eye area of Saints hooker Dylan Hartley after half an hour of the European clash at the Cardiff City Stadium.
Rees pleaded not guilty to the complaint on the basis that any contact had been accidental.
The independent judicial officer ruled that Rees was guilty of foul play but found that the contact had been reckless rather than 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, the judicial officer suspended Rees up until March 31.
Although both the Blues and ERC have the right to appeal the decision, the current length of suspension would prevent Rees from featuring in the Magners League until Round 19.
Rees would miss the games against Aironi (for which he wasn't selected due to the hearing), Glasgow, Leinster, Ulster, Edinburgh and Munster, with his first possible game back being the trip to the Ospreys on the weekend of April 1.
The length of the suspension also rules Rees out of the entire Six Nations tournament, with Wales playing their final match against France in Paris some 12 days before the suspension comes to an end.
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