Premiership bosses had already told its clubs that they could sign out-of-contract players, despite the fact that this was in clear violation of the FIFA rule.
The rule change means that Boro's recent signing of Tony Vidmar is now legitimate, not that it was ever in question by the Premier League.
FIFA accepted that due to the current financial climate they would relax the rule for the good of the players, in a letter to all its member associations FIFA explained "The strict application of the transfer registration periods would cause serious inconvenience to players still looking for a club, since on the one hand they would have no income and on the other they would not be allowed to register for a new club when the official transfer window has closed."
Players must be out of contract when the transfer window closes, clubs will not be allowed to sign players who have their contracts terminated by another club, or who resign from another club, between the transfer windows.