FIFA's medical advisers say new designs are needed because headscarfs worn by Muslim women players could lead to head and neck injuries.
Michel D'Hooghe, chairman of FIFA's medical committee, says samples received from two designers "represented a danger" to players' health.
Soccer's rules-making panel has said it intends to approve headscarves following a campaign by FIFA executive committee member Prince Ali of Jordan.
However, the panel asked for more medical advice before it meets on July 2.
D'Hooghe says his group of doctors tested the scarves and believed they could lead to head and neck injuries, plus overheating.
Modified designs are expected to arrive at FIFA "in the coming days," the Belgian doctor said.