Prosecutors tell TMZ Sports they believe Aldon Smith was driving with a BAC of greater than .20 before he was arrested for a DUI earlier this month.
Smith was booked back on Dec. 6 after cops say he had driven drunk in San Mateo County and then got involved in a two-car accident -- and, on Monday, prosecutors formally filed charges against the 32-year-old over it all.
Court documents show Smith was hit with two felony DUI charges ... and, in the docs, prosecutors revealed they believe Smith was HEAVILY intoxicated at the time of the incident.