The reports in the national press claim that Birmingham City have failed to register defender Kristian Pedersen to play, despite signing the full-back last month.
The club must still pay his wages, but could be punished if he features for them in a competitive match.
Blues spent heavily last summer with Harry Redknapp at the helm, bringing in the likes of Jota, Marc Roberts, Maxime Colin and Harlee Dean at St Andrews.
However, after a poor start to the season, Redknapp was axed and replaced by Steve Cotterill, who also left the club ahead of Garry Monk's arrival later in the campaign.
It's this spending spree which has reportedly brought on the embargo, which has the potential to throw Monk's remaining transfer plans into disarray if not solved quickly.
Monk is keen to bring in new faces, whilst also moving the likes of David Stockdale, Diego Fabbrini and Jonathan Grounds out ahead of the start of the season.
If no solution is found, the former Middlesbrough boss may be forced to play with the hand he has until the reported embargo is lifted.
Blues have yet to issue a statement on the reports.
If confirmed, Blues would join bitter rivals Aston Villa in finding themselves in financial struggles this summer, with the Villans' problems being well-documented since their failure to secure promotion via the play-offs.