Mercedes team boss Ross Brawn insists he checked with the FIA prior to the test.
Mercedes' rivals allege the German squad conducted an illegal three-day test with Pirelli at Barcelona, just after the recent Spanish Grand Prix.Red Bull's Christian Horner blasted.
The FIA summoned all of the parties to hearings in the Monaco stewards room in the immediate aftermath of the Grand Prix.
It appears Pirelli's right to conduct a mid-season test, and the strict in-season testing ban, have collided in a cauldron of typical Formula One politics.
Mercedes team boss Ross Brawn insists he checked with the FIA prior to the test, "and they were happy".
Horner, however, insists: "They have run a current car with current drivers and current tyres. That's a clear breach of the sporting regulations."
A Ferrari spokesman agreed that while Pirelli is allowed to test, it has to be with a two-year old car.
"The fundamental aspect is the year of the car," he said, "because if you use a current car it should be allegedly a breach".