Formula One's governing body is to appeal against the decision by the San Marino Grand Prix stewards not to take any action against Jenson Button.
Officials found Button's BAR car was underweight without fuel after Sunday's race in which the Briton came third.
They accepted BAR's explanation for the discrepancy - which has not been made public - but the FIA plans to examine the case at a Paris hearing on 4 May.
Cars must always comply with the rules - including a minimum weight limit.
F1's minimum weight limit is 600kg after the race and Button's car was 606.1kg when it was weighed on Sunday afternoon with the driver's weight included.
But it weighed only 594.6kg when the car was drained of fuel and it took race stewards nearly six hours to conclude that the car was legal.
An FIA statement on Monday said: "The FIA is to appeal against the decision of the stewards at the 2005 San Marino Grand Prix to take no further action in respect of car number three.
"The International Court of Appeal is scheduled to hear the case on 4 May."
BAR said they were "surprised" by the FIA's decision.
The team said in a statement that the car was found to be "compliant with all the regulations, including being above the weight limit" after the FIA performed its usual checks after the race.
"Thereafter, the FIA decided to perform a further check and requested that the team drain the fuel tank and then siphon out all the residual fuel remaining in the system," the team added.
"Once this had been completed, they determined that the car was below the required minimum weight.
"The FIA invited BAR-Honda to explain their position and the team was able to demonstrate, using its own data and data gathered by the investigating stewards, that the car was above minimum weight at all times during the race.
"Late last night, at the Imola circuit, the stewards confirmed that they were satisfied with the factual data supplied by the team."
BAR chief executive Nick Fry said: "BAR Honda will provide the same rigorous data presented to the stewards last night and is confident that it can prove once again that the car was fully compliant with the FIA technical regulations throughout the San Marino Grand Prix."