Ulster lock Dan Tuohy will not be cited for the high tackle that hospitalised Leicester and England fly-half Toby Flood.
Flood was knocked out during Leicester's pre-season victory at Welford Road last weekend and spent the night in hospital where scans revealed no further damage.
However, the 28-year-old Leicester captain will miss his side's Aviva Premiership opener against Worcester at Welford Road on Sunday on medical advice.
Tuohy received a yellow card for the offence but the RFU have revealed they would be unable to take any further action due to a disciplinary loophole - that they will see now seek to close - due to the game's pre-season status.
"Ulster players were not subject to the RFU's jurisdiction in this match and as a result no action can be taken," an RFU spokesperson said.
"Clubs can agree how discipline should be dealt with in advance of such games and we're looking to see how we can make sure this always happens in the future."
Tuohy escaping punishment will further infuriate Leicester's director of rugby Richard Cockerill, who earlier this week asked for the incident to be investigated.
"It's these sort of things that will stop little Johnny coming to mini-rugby on a Sunday morning," Cockerill said.
"Once again, our playmaker has been hit and we have lost him from the game. It is illegal.
"You would expect the powers-that-be to have a look at that."