F1's governing body has declared illegal the 'holes' in the floor of Red Bull's 2012 car.
Rivals teams including Ferrari, McLaren and Mercedes had raised concerns about the design at Monaco last weekend, but opted not to protest on the promise that the FIA would issue a clarification prior to Canada next weekend.
"I think the regulation is quite a grey area," Red Bull's Christian Horner acknowledged to Sky Sports, "and I think a clarification will come out before Montreal that will tidy it up through a technical directive."
The crux of the highly-anticipated technical directive was reported by the German media on Saturday.
Named 013-12, the directive reportedly said there is a "misunderstanding" on the basis of a technical regulation about the meaning of a "fully enclosed hole".
It concluded that the design featured on Mark Webber's winning Monaco car is in fact not allowed.
The team and Australian Webber will keep the win, but the RB8 will have to be modified in order to pass scrutineering next weekend in Montreal.