McLaren engineering director Tim Goss has admitted that the ban on off-throttle blown diffusers would hurt the performance of his team.
Goss is adamant, however, that it will also be a setback for McLaren's main rivals too.
After the Turkish GP, the FIA issued a rules clarification that would have banned off-throttle blown diffusers, which use engine mapping settings to keep gases pumping through the diffuser even when the driver is not pushing the throttle.
The governing body, however, said on Tuesday evening that it had decided not to go through with the change to the diffuser regulations for this weekend's Spanish GP.
However, the FIA is believed to be keen for the off-throttle usage to be stamped out as soon as possible.
Goss, speaking during a Vodafone phone-in teleconference on Wednesday, admitted that his team would be affected by the changes.
"It's difficult to know," said Goss about how big an effect the changes would have in the performance of McLaren's car.
"All the teams are up to the same tricks with regards engine mapping. Certainly we exploit them.
"The latest set of guidelines the FIA has given us regarding engine mapping would be a performance setback for us if and when they come in. I know it would almost certainly be a performance setback to our major competitors.
"Whether it affects us more than our major competitors is something I don't know. I know what we get out of it and it's a substantial benefit. But I imagine it will be just as sizeable a setback to our competitors.
"We're just working to the latest set of FIA guidelines. We can react to whatever they tell us reasonably promptly, and for the moment it would appear that the FIA considers it quite a complex matter and they have to go back and consider exactly how they police it.
"So as a result it looks like in Spain it will be business as usual."
Goss admitted he did not know the reasons why the FIA had decide to take the decision to change the rules now.
"I don't know the background. I don't know if they've taken it upon themselves to clamp down on this, or whether somebody's prompted them to.
"Since mid 2010 it's become apparent to journalists and then the rest of the public that teams have been changing engine maps to get more out of the exhaust momentum and the effects of that on the rear of the car. This been around for a while, but there's been nothing mentioned about clamping down."