Northampton and England hooker Dylan Hartley has been cited over an incident during Friday's Heineken Cup defeat by Ulster.
Hartley will face an ERC disciplinary panel after striking Ulster forward Rory Best with his arm during the Pool 4 clash at Franklin's Gardens, which the visitors won 25-6.
The Saints skipper was reported by match commissioner Jean-Claude Legendre after the incident, which was also caught on camera.
Hartley has only just returned to action after suffering a knee injury that ruled him out of the entire autumn Test campaign, an absence that gave Tom Youngs a chance to shine.
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."
Hartley has a chequered disciplinary past including a 26-week ban for gouging the eyes of Wasps players James Haskell and Jonny O'Connor in 2007.
He was also banned for eight weeks earlier this year for biting Ireland flanker Stephen Ferris during the Six Nations clash at Twickenham.