Tuilagi punched England wing Ashton three times in the head during Leicester's Premiership semi-final victory over Northampton last Saturday.
The 20-year-old World Cup prospect will be suspended for Leicester's Premiership final against Saracens on May 28 and the England Saxons' entire Churchill Cup campaign.
But it could have been a whole lot worse for Tuilagi, who will be available for England's three World Cup warm-up internationals in August if selected in Martin Johnson's 50-man training squad.
Rugby Football Union disciplinary chief Judge Jeff Blackett described Tuilagi's actions as "very damaging to the image of the game" and worthy of a 10-week suspension.
Blackett warned Tuilagi he would have been prosecuted had the incident occurred off the field.
But the RFU's three-man disciplinary panel decided to cut the 10-week ban in half after taking a number of mitigating factors into account, including Tuilagi's age and guilty plea.
Ashton, who required stitches to a cut above the eye, was also deemed to have provoked Tuilagi into throwing the punches.
Blackett said: "This was a top-end entry offence because there were three punches, the third of which was extremely forceful and caused significant injury.
"However we also determined that there was some provocation from Chris Ashton who pushed and struck him in the back with his knee.
"The top-end range is eight to 52 weeks and we determined that the appropriate entry point within that range is 10 weeks.
"This was reduced by 50% to reflect Manu's youth and inexperience, his admission of guilt and his genuine remorse.
"This sort of incident is very damaging to the image of the game and there is no place for this type of offending on the rugby pitch.
"Had it occurred in the high street an offender would have been prosecuted in the criminal courts.
"Nevertheless we are confident that Manu Tuilagi will learn a valuable lesson from this."
The decision to pin some of the blame on Ashton is controversial, given the England wing had reacted to being taken out by a tackle from Tuilagi that appeared high and was certainly off the ball.
Both players were sin-binned for the incident by referee Wayne Barnes on the advice of touch-judge Robin Goodliffe, who had wrongly claimed Ashton had also been throwing punches.
The Grand Final and Churchill Cup would have given Tuilagi, the Premiership's young player of the year, a perfect opportunity to press his claims for World Cup selection.
Leicester team-mate Toby Flood said on Twitter: "It was coming but real shame for him to miss the final, he's had an outstanding season."
Johnson, the England manager, will name his World Cup training squad on June 17, a week before Tuilagi's ban expires.
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