Northampton and England hooker Dylan Hartley has been cited over an incident during Friday's Heineken Cup defeat by Ulster.
The Saints skipper has been reported by match commissioner Jean-Claude Legendre for striking opposite number and Ulster hooker Rory Best with his arm, European Rugby Cup announced.
No date has yet been set for Hartley's disciplinary hearing.
Hartley has only recently returned to action after suffering an injury that sidelined him from England's autumn Test campaign.
Northampton, meanwhile, are facing a pool stage exit from the Heineken Cup following a 25-6 defeat to Ulster.
The teams meet again in Belfast next Saturday, with Saints needing victory to keep alive their slim quarter-final hopes.
Recommended International Rugby Board sanctions for striking with the fist or arm range from a two-week ban at the low end to a maximum 52 weeks.
"Hartley is alleged to have struck Ulster Rugby forward Rory Best with his arm during the second half of the match at Franklin's Gardens," ERC said in a statement.
"Details of the hearing date and the appointed independent judicial officer will be announced shortly."
Hooker Hartley, 26, has a chequered past in terms of disciplinary issues.
He was banned for 26 weeks in April 2007 after being found guilty of eye-gouging Wasps players James Haskell and Jonny O'Connor.
It was a punishment that dashed his hopes of being part of England's World Cup campaign in France later that year.
And earlier this year, he was banned for eight weeks after being cited for biting Ireland flanker Stephen Ferris during an 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.
And it could also jeopardise his chances of reclaiming the England hooking berth that was impressively filled by Leicester's Tom Youngs during the autumn games.
England kick off their Six Nations campaign against Scotland at Twickenham on February 2.