Denmark striker Nicklas Bendtner could face sanction by UEFA after revealing the logo of a betting company while celebrating his second goal against Portugal in their Euro 2012 clash on Wednesday.
Pictures taken by a Reuters photographer show Bendtner lifting his shirt to show the name of an Irish firm on the waistband of his underpants.
Sponsors pay millions to be associated with the European Championship and governing body UEFA has strict rules preventing ambush marketing.
A spokesperson said UEFA was investigating the matter.
Bendtner scored twice in Denmark's 3-2 defeat by Portugal in Group B, prompting a spokesman for the betting company to release a statement praising his performance and claiming credit for Bendtner's good luck.
The spokesman also said no money had changed hands with Bendtner and that the underpants have been sent to several unnamed players so could make a reappearance in the tournament.
Denmark face Germany on Sunday in their final group game as they look to reach the last eight.
(Reporting by Phil O'Connor and Justin Palmer; Editing by Mark Meadows; firstname.lastname@example.org; +44 207 542 7933; Twitter: @mark_meadows; Reuters Messaging: email@example.com; For; the sports blog Left Field: blogs.reuters.com/sport)