Saints have been hit with a £60,000 fine for allowing George North to play for Wales outside of the IRB window.
North was given permission to take to the field for his country against Australia on November 30, the same day as Northampton beat Worcester in the Aviva Premiership.
Premiership clubs have an agreement in place that only England players can be released outside of the international window.
That led Premiership Rugby to haul Saints in front of a disciplinary panel, which decided the club should face a fine.
Northampton will not appeal against the decision.