All the Premiership clubs have signed an agreement stating that only players representing England can be released outside the window and Northampton admitted they had broken the rules regarding the Cardiff match on November 30, which Australia won 30-26.
The 21-year-old North, a key player in the British and Irish Lions team who won this year's series in Australia, joined Northampton from Welsh club Scarlets at the end of last season.
(Reporting by Mitch Phillips, editing by Ed Osmond)
