England forward Dylan Hartley has been cited for alleged foul play during Northampton's Heineken Cup defeat against Ulster on Friday night.
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."