The 29-year-old, capped 39 times by England, is alleged to have bitten Northampton prop Alex Waller during Saturday's 27-12 Premiership victory for champions Saracens.
The two incidents occurred in the 24th and 25th minutes of the clash at Saracens' Allianz Park ground in north London, with Ashton's mouth appearing to make contact with Waller's hand during a clash on the floor.