Northampton have been fined £60,000 by a Premiership Rugby disciplinary panel for letting wing George North play for Wales against Australia on 30 November.
The match, which Wales lost 30-26, took place outside of the International Rugby Board's autumn window.
Premiership clubs have agreed that only players representing England can be released for matches outside of that international window.
Saints admitted the breach and will not appeal against the sanction.
The club have previously said an agreement was made with North, 21, when he signed to "allow him to represent Wales if and when selected".
Northampton are the first team to contravene the Premiership Rugby policy, but the verdict could affect other players, with North's Wales team-mate Richard Hibbard joining Gloucester next season.
The Welsh Rugby Union has again scheduled a fourth match next autumn, against South Africa, which falls outside the IRB window.