Northampton hooker Dylan Hartley has been cited for striking Ulster forward Rory Best with his arm during their Heineken Cup match on Friday.
The Saints skipper faces a disciplinary hearing in Dublin on Thursday, following the incident which occurred during the second half of the game.
at their Franklin's Gardens ground.
The defeat leaves them third in Pool 4, with the return trip to Ulster's Ravenhill stadium to come on Saturday.
Ulster are top of the pool, having won all three of their matches, and Saints require victory to keep their slim quarter-final hopes alive.
Hartley has only recently returned to action that sidelined him from England's autumn Test campaign.
The recommended International Rugby Board sanctions for striking with the fist or arm range from a two-week ban to a maximum 52 weeks.
Hartley, 26, was banned for 26 weeks in April 2007 after being found guilty of eye-gouging Wasps players James Haskell and Jonny O'Connor.
Earlier this year, he was banned for eight weeks after being cited for biting Ireland flanker Stephen Ferris during a Six Nations clash at Twickenham.
If Hartley is banned for the Best incident, he could miss forthcoming Aviva Premiership and Heineken Cup games for Northampton.
It could also jeopardise his chances of reclaiming the England hooking berth that was impressively filled by Leicester's Tom Youngs during the Autumn Internationals.
England kick off their Six Nations campaign against Scotland at Twickenham on 2 February.