The 19-year-old England Saxons cap, a contender for a place in England's World Cup squad, caught Saints star Ashton with a three-punch combination in the 32nd minute of Leicester's 11-3 top-flight semi-final success at Welford Road on Saturday.
Tuilagi, the Premiership Young Player of the Year, should have been shown a red card for but, instead, both players were sin-binned on the advice of assistant referee Robin Goodliffe.
However, independent citing officer Alan Mansell has decided that Tuilagi has a case to answer and he will now appear before a Rugby Football Union disciplinary hearing tomorrow, on his 20th birthday, which will be chaired by Judge Jeff Blackett in London.
A suspension could range from three to 11 weeks or more, and would end Tuilagi's hopes of appearing against Saracens in the Twickenham showpiece on May 28.
Tigers' director of rugby Richard Cockerill said: "He reacted poorly to something that had happened, he has thrown a punch and you can't do that. The rules say that, if you strike or punch a player, then it is a red card offence, make of that what you will. I am sure there will be some form of sanction."
Tuilagi has been one of the stars of the English season, notably running in a wonderful solo score in Leicester's home defeat to Sarries earlier this season but, while Cockerill acknowledges it will be a blow to be without the powerful three-quarter, he believes he has plenty of quality back-up to call upon.
"If it takes him out of the final we have to deal with it," he said.
"Manu is a good player, a quality player, but so are Dan Hipkiss, Matty Smith and Billy Twelvetrees, so there are other things we can do and other players we can pick."
Cockerill also hit back at newspaper reports that he had used foul and abusive language towards RFU referee assessor Brian Campsall during the course of the uncompromising last-four clash.
It had been suggested that Cockerill, who received a warning from Judge Blackett last year over his behaviour during a play-off win over Bath, and head coach Matt O'Connor would face disciplinary action for the manner in which they voiced criticism of the performance of international referee Wayne Barnes.
But Cockerill, seeking a hat-trick of league title as Tigers boss, revealed that he has discussed the matter with Campsall and RFU referees' chief Ed Morrison.
He said: "I want to state categorically that I did not use foul and abusive language and I was not aggressive towards Brian."