Andy Webster's future at Rangers is in doubt after the Scottish Football Association confirmed Hearts have refused to authorise his registration.
Hearts contacted the SFA with their concerns about the loan transfer of Webster from Wigan to Rangers, and the issue has been passed into FIFA hands.
When Webster left Hearts last summer, he did so by taking advantage of article 17 of FIFA's regulations which permitted him to walk out 12 months before the end of a four-year contract, providing he moved to another country.
However Webster, 24, remains registered with Hearts at the SFA, and the Tynecastle club claim he should not be allowed to join Rangers until his original contract with Hearts expires.
That will elapse at the end of June, however Rangers were planning to play Webster - who moved to Glasgow on January 5 - as soon as he recovers from a strained medial ligament injury.
The SFA confirmed: 'We are unable to register Andy Webster for Rangers as he is already registered at Hearts. The matter has been passed to FIFA.'
The world body will decide Webster's future, with Hearts concerned he could be playing again in the SPL so soon after departing on the proviso he was joining an English club.
Wigan and Hearts have yet to agree on a compensation package for Webster's summer transfer.